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Re: [PATCH v5 2/3] target/riscv: Add stimecmp support
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From: |
Alistair Francis |
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Subject: |
Re: [PATCH v5 2/3] target/riscv: Add stimecmp support |
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Date: |
Thu, 21 Jul 2022 10:04:21 +1000 |
On Thu, Jul 21, 2022 at 5:32 AM Atish Patra <atishp@atishpatra.org> wrote:
>
> On Wed, Jun 15, 2022 at 8:25 PM Anup Patel <anup@brainfault.org> wrote:
> >
> > On Thu, Jun 16, 2022 at 8:08 AM Alistair Francis <alistair23@gmail.com>
> > wrote:
> > >
> > > On Thu, Jun 16, 2022 at 4:21 AM Atish Kumar Patra <atishp@rivosinc.com>
> > > wrote:
> > > >
> > > > On Wed, Jun 8, 2022 at 12:19 AM Alistair Francis <alistair23@gmail.com>
> > > > wrote:
> > > > >
> > > > > On Mon, Jun 6, 2022 at 2:23 AM Atish Patra <atishp@atishpatra.org>
> > > > > wrote:
> > > > > >
> > > > > > On Thu, Jun 2, 2022 at 12:02 AM Alistair Francis
> > > > > > <alistair23@gmail.com> wrote:
> > > > > > >
> > > > > > > On Wed, Jun 1, 2022 at 4:16 AM Atish Patra <atishp@rivosinc.com>
> > > > > > > wrote:
> > > > > > > >
> > > > > > > > stimecmp allows the supervisor mode to update stimecmp CSR
> > > > > > > > directly
> > > > > > > > to program the next timer interrupt. This CSR is part of the
> > > > > > > > Sstc
> > > > > > > > extension which was ratified recently.
> > > > > > > >
> > > > > > > > Signed-off-by: Atish Patra <atishp@rivosinc.com>
> > > > > > > > ---
> > > > > > > > target/riscv/cpu.c | 8 ++++
> > > > > > > > target/riscv/cpu.h | 5 ++
> > > > > > > > target/riscv/cpu_bits.h | 4 ++
> > > > > > > > target/riscv/csr.c | 81 +++++++++++++++++++++++++++++++
> > > > > > > > target/riscv/machine.c | 1 +
> > > > > > > > target/riscv/meson.build | 3 +-
> > > > > > > > target/riscv/time_helper.c | 98
> > > > > > > > ++++++++++++++++++++++++++++++++++++++
> > > > > > > > target/riscv/time_helper.h | 30 ++++++++++++
> > > > > > > > 8 files changed, 229 insertions(+), 1 deletion(-)
> > > > > > > > create mode 100644 target/riscv/time_helper.c
> > > > > > > > create mode 100644 target/riscv/time_helper.h
> > > > > > > >
> > > > > > > > diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> > > > > > > > index 19f4e8294042..d58dd2f857a7 100644
> > > > > > > > --- a/target/riscv/cpu.c
> > > > > > > > +++ b/target/riscv/cpu.c
> > > > > > > > @@ -23,6 +23,7 @@
> > > > > > > > #include "qemu/log.h"
> > > > > > > > #include "cpu.h"
> > > > > > > > #include "internals.h"
> > > > > > > > +#include "time_helper.h"
> > > > > > > > #include "exec/exec-all.h"
> > > > > > > > #include "qapi/error.h"
> > > > > > > > #include "qemu/error-report.h"
> > > > > > > > @@ -779,7 +780,12 @@ static void riscv_cpu_init(Object *obj)
> > > > > > > > #ifndef CONFIG_USER_ONLY
> > > > > > > > qdev_init_gpio_in(DEVICE(cpu), riscv_cpu_set_irq,
> > > > > > > > IRQ_LOCAL_MAX + IRQ_LOCAL_GUEST_MAX);
> > > > > > > > +
> > > > > > > > + if (cpu->cfg.ext_sstc) {
> > > > > > > > + riscv_timer_init(cpu);
> > > > > > > > + }
> > > > > > > > #endif /* CONFIG_USER_ONLY */
> > > > > > > > +
> > > > > > > > }
> > > > > > > >
> > > > > > > > static Property riscv_cpu_properties[] = {
> > > > > > > > @@ -806,6 +812,7 @@ static Property riscv_cpu_properties[] = {
> > > > > > > > DEFINE_PROP_BOOL("mmu", RISCVCPU, cfg.mmu, true),
> > > > > > > > DEFINE_PROP_BOOL("pmp", RISCVCPU, cfg.pmp, true),
> > > > > > > > DEFINE_PROP_BOOL("debug", RISCVCPU, cfg.debug, true),
> > > > > > > > + DEFINE_PROP_BOOL("sstc", RISCVCPU, cfg.ext_sstc, true),
> > > > > > >
> > > > > > > Do we want this enabled by default?
> > > > > > >
> > > > > >
> > > > > > sstc extension will result in performance improvements as it avoids
> > > > > > the SBI calls & interrupt forwarding
> > > > > > path. That's why I think it should be enabled by default.
> > > > > >
> > > > > > > >
> > > > > > > > DEFINE_PROP_STRING("priv_spec", RISCVCPU, cfg.priv_spec),
> > > > > > > > DEFINE_PROP_STRING("vext_spec", RISCVCPU, cfg.vext_spec),
> > > > > > > > @@ -965,6 +972,7 @@ static void riscv_isa_string_ext(RISCVCPU
> > > > > > > > *cpu, char **isa_str, int max_str_len)
> > > > > > > > ISA_EDATA_ENTRY(zbs, ext_zbs),
> > > > > > > > ISA_EDATA_ENTRY(zve32f, ext_zve32f),
> > > > > > > > ISA_EDATA_ENTRY(zve64f, ext_zve64f),
> > > > > > > > + ISA_EDATA_ENTRY(sstc, ext_sstc),
> > > > > > > > ISA_EDATA_ENTRY(svinval, ext_svinval),
> > > > > > > > ISA_EDATA_ENTRY(svnapot, ext_svnapot),
> > > > > > > > ISA_EDATA_ENTRY(svpbmt, ext_svpbmt),
> > > > > > > > diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
> > > > > > > > index 1119d5201066..9a5e02f217ba 100644
> > > > > > > > --- a/target/riscv/cpu.h
> > > > > > > > +++ b/target/riscv/cpu.h
> > > > > > > > @@ -276,6 +276,9 @@ struct CPUArchState {
> > > > > > > > uint64_t mfromhost;
> > > > > > > > uint64_t mtohost;
> > > > > > > >
> > > > > > > > + /* Sstc CSRs */
> > > > > > > > + uint64_t stimecmp;
> > > > > > > > +
> > > > > > > > /* physical memory protection */
> > > > > > > > pmp_table_t pmp_state;
> > > > > > > > target_ulong mseccfg;
> > > > > > > > @@ -329,6 +332,7 @@ struct CPUArchState {
> > > > > > > > float_status fp_status;
> > > > > > > >
> > > > > > > > /* Fields from here on are preserved across CPU reset. */
> > > > > > > > + QEMUTimer *stimer; /* Internal timer for S-mode interrupt
> > > > > > > > */
> > > > > > > >
> > > > > > > > hwaddr kernel_addr;
> > > > > > > > hwaddr fdt_addr;
> > > > > > > > @@ -379,6 +383,7 @@ struct RISCVCPUConfig {
> > > > > > > > bool ext_counters;
> > > > > > > > bool ext_ifencei;
> > > > > > > > bool ext_icsr;
> > > > > > > > + bool ext_sstc;
> > > > > > > > bool ext_svinval;
> > > > > > > > bool ext_svnapot;
> > > > > > > > bool ext_svpbmt;
> > > > > > > > diff --git a/target/riscv/cpu_bits.h b/target/riscv/cpu_bits.h
> > > > > > > > index 4e5b630f5965..29d0e4a1be01 100644
> > > > > > > > --- a/target/riscv/cpu_bits.h
> > > > > > > > +++ b/target/riscv/cpu_bits.h
> > > > > > > > @@ -215,6 +215,10 @@
> > > > > > > > #define CSR_STVAL 0x143
> > > > > > > > #define CSR_SIP 0x144
> > > > > > > >
> > > > > > > > +/* Sstc supervisor CSRs */
> > > > > > > > +#define CSR_STIMECMP 0x14D
> > > > > > > > +#define CSR_STIMECMPH 0x15D
> > > > > > > > +
> > > > > > > > /* Supervisor Protection and Translation */
> > > > > > > > #define CSR_SPTBR 0x180
> > > > > > > > #define CSR_SATP 0x180
> > > > > > > > diff --git a/target/riscv/csr.c b/target/riscv/csr.c
> > > > > > > > index 245f007e66e1..48d07911ae14 100644
> > > > > > > > --- a/target/riscv/csr.c
> > > > > > > > +++ b/target/riscv/csr.c
> > > > > > > > @@ -21,6 +21,7 @@
> > > > > > > > #include "qemu/log.h"
> > > > > > > > #include "qemu/timer.h"
> > > > > > > > #include "cpu.h"
> > > > > > > > +#include "time_helper.h"
> > > > > > > > #include "qemu/main-loop.h"
> > > > > > > > #include "exec/exec-all.h"
> > > > > > > > #include "sysemu/cpu-timers.h"
> > > > > > > > @@ -537,6 +538,76 @@ static RISCVException
> > > > > > > > read_timeh(CPURISCVState *env, int csrno,
> > > > > > > > return RISCV_EXCP_NONE;
> > > > > > > > }
> > > > > > > >
> > > > > > > > +static RISCVException sstc(CPURISCVState *env, int csrno)
> > > > > > > > +{
> > > > > > > > + CPUState *cs = env_cpu(env);
> > > > > > > > + RISCVCPU *cpu = RISCV_CPU(cs);
> > > > > > > > +
> > > > > > > > + if (!cpu->cfg.ext_sstc || !env->rdtime_fn) {
> > > > > > > > + return RISCV_EXCP_ILLEGAL_INST;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + if (env->priv == PRV_M) {
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + if (env->priv != PRV_S) {
> > > > > > > > + return RISCV_EXCP_ILLEGAL_INST;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + /*
> > > > > > > > + * No need of separate function for rv32 as menvcfg stores
> > > > > > > > both menvcfg
> > > > > > > > + * menvcfgh for RV32.
> > > > > > > > + */
> > > > > > > > + if (!(get_field(env->mcounteren, COUNTEREN_TM) &&
> > > > > > > > + get_field(env->menvcfg, MENVCFG_STCE))) {
> > > > > > > > + return RISCV_EXCP_ILLEGAL_INST;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +static RISCVException read_stimecmp(CPURISCVState *env, int
> > > > > > > > csrno,
> > > > > > > > + target_ulong *val)
> > > > > > > > +{
> > > > > > > > + *val = env->stimecmp;
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +static RISCVException read_stimecmph(CPURISCVState *env, int
> > > > > > > > csrno,
> > > > > > > > + target_ulong *val)
> > > > > > > > +{
> > > > > > > > + *val = env->stimecmp >> 32;
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +static RISCVException write_stimecmp(CPURISCVState *env, int
> > > > > > > > csrno,
> > > > > > > > + target_ulong val)
> > > > > > > > +{
> > > > > > > > + RISCVCPU *cpu = env_archcpu(env);
> > > > > > > > +
> > > > > > > > + if (riscv_cpu_mxl(env) == MXL_RV32) {
> > > > > > > > + env->stimecmp = deposit64(env->stimecmp, 0, 32,
> > > > > > > > (uint64_t)val);
> > > > > > > > + } else {
> > > > > > > > + env->stimecmp = val;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + riscv_timer_write_timecmp(cpu, env->stimer, env->stimecmp,
> > > > > > > > 0, MIP_STIP);
> > > > > > > > +
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +static RISCVException write_stimecmph(CPURISCVState *env, int
> > > > > > > > csrno,
> > > > > > > > + target_ulong val)
> > > > > > > > +{
> > > > > > > > + RISCVCPU *cpu = env_archcpu(env);
> > > > > > > > +
> > > > > > > > + env->stimecmp = deposit64(env->stimecmp, 32, 32,
> > > > > > > > (uint64_t)val);
> > > > > > > > + riscv_timer_write_timecmp(cpu, env->stimer, env->stimecmp,
> > > > > > > > 0, MIP_STIP);
> > > > > > > > +
> > > > > > > > + return RISCV_EXCP_NONE;
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > /* Machine constants */
> > > > > > > >
> > > > > > > > #define M_MODE_INTERRUPTS ((uint64_t)(MIP_MSIP | MIP_MTIP |
> > > > > > > > MIP_MEIP))
> > > > > > > > @@ -1515,6 +1586,12 @@ static RISCVException
> > > > > > > > rmw_mip64(CPURISCVState *env, int csrno,
> > > > > > > > new_val |= env->external_seip * MIP_SEIP;
> > > > > > > > }
> > > > > > > >
> > > > > > > > + if (cpu->cfg.ext_sstc && (env->priv == PRV_M) &&
> > > > > > > > + get_field(env->menvcfg, MENVCFG_STCE)) {
> > > > > > > > + /* sstc extension forbids STIP & VSTIP to be writeable
> > > > > > > > in mip */
> > > > > > > > + mask = mask & ~(MIP_STIP | MIP_VSTIP);
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > if (mask) {
> > > > > > > > old_mip = riscv_cpu_update_mip(cpu, mask, (new_val &
> > > > > > > > mask));
> > > > > > > > } else {
> > > > > > > > @@ -3341,6 +3418,10 @@ riscv_csr_operations
> > > > > > > > csr_ops[CSR_TABLE_SIZE] = {
> > > > > > > > [CSR_SCAUSE] = { "scause", smode, read_scause,
> > > > > > > > write_scause },
> > > > > > > > [CSR_STVAL] = { "stval", smode, read_stval,
> > > > > > > > write_stval },
> > > > > > > > [CSR_SIP] = { "sip", smode, NULL, NULL,
> > > > > > > > rmw_sip },
> > > > > > > > + [CSR_STIMECMP] = { "stimecmp", sstc, read_stimecmp,
> > > > > > > > write_stimecmp,
> > > > > > > > + .min_priv_ver =
> > > > > > > > PRIV_VERSION_1_12_0 },
> > > > > > > > + [CSR_STIMECMPH] = { "stimecmph", sstc, read_stimecmph,
> > > > > > > > write_stimecmph,
> > > > > > > > + .min_priv_ver =
> > > > > > > > PRIV_VERSION_1_12_0 },
> > > > > > > >
> > > > > > > > /* Supervisor Protection and Translation */
> > > > > > > > [CSR_SATP] = { "satp", smode, read_satp,
> > > > > > > > write_satp },
> > > > > > > > diff --git a/target/riscv/machine.c b/target/riscv/machine.c
> > > > > > > > index 7d85de0b1d49..ee02bfc18916 100644
> > > > > > > > --- a/target/riscv/machine.c
> > > > > > > > +++ b/target/riscv/machine.c
> > > > > > > > @@ -334,6 +334,7 @@ const VMStateDescription vmstate_riscv_cpu
> > > > > > > > = {
> > > > > > > > VMSTATE_UINTTL(env.mscratch, RISCVCPU),
> > > > > > > > VMSTATE_UINT64(env.mfromhost, RISCVCPU),
> > > > > > > > VMSTATE_UINT64(env.mtohost, RISCVCPU),
> > > > > > > > + VMSTATE_UINT64(env.stimecmp, RISCVCPU),
> > > > > > > >
> > > > > > > > VMSTATE_END_OF_LIST()
> > > > > > > > },
> > > > > > > > diff --git a/target/riscv/meson.build b/target/riscv/meson.build
> > > > > > > > index 2c20f3dd8e9c..1243d019148e 100644
> > > > > > > > --- a/target/riscv/meson.build
> > > > > > > > +++ b/target/riscv/meson.build
> > > > > > > > @@ -29,7 +29,8 @@ riscv_softmmu_ss.add(files(
> > > > > > > > 'pmp.c',
> > > > > > > > 'debug.c',
> > > > > > > > 'monitor.c',
> > > > > > > > - 'machine.c'
> > > > > > > > + 'machine.c',
> > > > > > > > + 'time_helper.c'
> > > > > > > > ))
> > > > > > > >
> > > > > > > > target_arch += {'riscv': riscv_ss}
> > > > > > > > diff --git a/target/riscv/time_helper.c
> > > > > > > > b/target/riscv/time_helper.c
> > > > > > > > new file mode 100644
> > > > > > > > index 000000000000..f3fb5eac7b7b
> > > > > > > > --- /dev/null
> > > > > > > > +++ b/target/riscv/time_helper.c
> > > > > > > > @@ -0,0 +1,98 @@
> > > > > > > > +/*
> > > > > > > > + * RISC-V timer helper implementation.
> > > > > > > > + *
> > > > > > > > + * Copyright (c) 2022 Rivos Inc.
> > > > > > > > + *
> > > > > > > > + * This program is free software; you can redistribute it
> > > > > > > > and/or modify it
> > > > > > > > + * under the terms and conditions of the GNU General Public
> > > > > > > > License,
> > > > > > > > + * version 2 or later, as published by the Free Software
> > > > > > > > Foundation.
> > > > > > > > + *
> > > > > > > > + * This program is distributed in the hope it will be useful,
> > > > > > > > but WITHOUT
> > > > > > > > + * ANY WARRANTY; without even the implied warranty of
> > > > > > > > MERCHANTABILITY or
> > > > > > > > + * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
> > > > > > > > Public License for
> > > > > > > > + * more details.
> > > > > > > > + *
> > > > > > > > + * You should have received a copy of the GNU General Public
> > > > > > > > License along with
> > > > > > > > + * this program. If not, see <http://www.gnu.org/licenses/>.
> > > > > > > > + */
> > > > > > > > +
> > > > > > > > +#include "qemu/osdep.h"
> > > > > > > > +#include "qemu/log.h"
> > > > > > > > +#include "cpu_bits.h"
> > > > > > > > +#include "time_helper.h"
> > > > > > > > +#include "hw/intc/riscv_aclint.h"
> > > > > > > > +
> > > > > > > > +static void riscv_stimer_cb(void *opaque)
> > > > > > > > +{
> > > > > > > > + RISCVCPU *cpu = opaque;
> > > > > > > > + riscv_cpu_update_mip(cpu, MIP_STIP, BOOL_TO_MASK(1));
> > > > > > > > +}
> > > > > > > > +
> > > > > > > > +/*
> > > > > > > > + * Called when timecmp is written to update the QEMU timer or
> > > > > > > > immediately
> > > > > > > > + * trigger timer interrupt if mtimecmp <= current timer value.
> > > > > > > > + */
> > > > > > > > +void riscv_timer_write_timecmp(RISCVCPU *cpu, QEMUTimer *timer,
> > > > > > > > + uint64_t timecmp, uint64_t
> > > > > > > > delta,
> > > > > > > > + uint32_t timer_irq)
> > > > > > > > +{
> > > > > > > > + uint64_t diff, ns_diff, next
> > > > > > > > + CPURISCVState *env = &cpu->env;
> > > > > > > > + RISCVAclintMTimerState *mtimer = env->rdtime_fn_arg;
> > > > > > > > + uint32_t timebase_freq = mtimer->timebase_freq;
> > > > > > > > + uint64_t rtc_r = env->rdtime_fn(env->rdtime_fn_arg) +
> > > > > > > > delta;
> > > > > > >
> > > > > > > Can't we just pass mtimer here directly?
> > > > > > >
> > > > > >
> > > > > > Yeah. rank's recent patch changed the argument to mtimer instead of
> > > > > > timebase_freq.
> > > > > > I forgot to change here while rebasing. I will change this.
> > > > > >
> > > > > > > Ok, so I realise this is based on the aclint, but I've now
> > > > > > > confused myself.
> > > > > > >
> > > > > > > So rtc_r is the current time in ticks
> > > > > > >
> > > > > > > > +
> > > > > > > > + if (timecmp <= rtc_r) {
> > > > > > > > + /*
> > > > > > > > + * If we're setting an stimecmp value in the "past",
> > > > > > > > + * immediately raise the timer interrupt
> > > > > > > > + */
> > > > > > > > + riscv_cpu_update_mip(cpu, timer_irq, BOOL_TO_MASK(1));
> > > > > > > > + return;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + /* Clear the [V]STIP bit in mip */
> > > > > > > > + riscv_cpu_update_mip(cpu, timer_irq, BOOL_TO_MASK(0));
> > > > > > > > +
> > > > > > > > + /* otherwise, set up the future timer interrupt */
> > > > > > > > + diff = timecmp - rtc_r;
> > > > > > >
> > > > > > > timecmp is the absolute time we want the interrupt in ticks
> > > > > > >
> > > > > > > > + /* back to ns (note args switched in muldiv64) */
> > > > > > > > + ns_diff = muldiv64(diff, NANOSECONDS_PER_SECOND,
> > > > > > > > timebase_freq);
> > > > > > >
> > > > > > > ns_diff is the amount of ns in the future we want the interrupt
> > > > > > > to occur
> > > > > > >
> > > > > > > > +
> > > > > > > > + /*
> > > > > > > > + * check if ns_diff overflowed and check if the addition
> > > > > > > > would potentially
> > > > > > > > + * overflow
> > > > > > > > + */
> > > > > > > > + if ((NANOSECONDS_PER_SECOND > timebase_freq && ns_diff <
> > > > > > > > diff) ||
> > > > > > > > + ns_diff > INT64_MAX) {
> > > > > > > > + next = INT64_MAX;
> > > > > > > > + } else {
> > > > > > > > + /*
> > > > > > > > + * as it is very unlikely qemu_clock_get_ns will
> > > > > > > > return a value
> > > > > > > > + * greater than INT64_MAX, no additional check is
> > > > > > > > needed for an
> > > > > > > > + * unsigned integer overflow.
> > > > > > >
> > > > > > > I don't think qemu_clock_get_ns() can return a value larger than
> > > > > > > INT64_MAX
> > > > > > >
> > > > > >
> > > > > > Yup as the return type is int64_t :)
> > > > > >
> > > > > > > > + */
> > > > > > > > + next = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL) + ns_diff;
> > > > > > >
> > > > > > > next is the absolute time (in ns) that we want the interrupt to
> > > > > > > occur.
> > > > > > >
> > > > > > > So we end up performing
> > > > > > >
> > > > > > > ((timecmp - now_in_ticks) in ns) + now_in_ns
> > > > > > >
> > > > > > > Can't we just convert timecmp to ns and then set that directly
> > > > > > > with timer_mod()?
> > > > > > >
> > > > > >
> > > > > > That should work for all the case except
> > > > > > if ((NANOSECONDS_PER_SECOND > timebase_freq && ns_diff < diff)
> > > > > >
> > > > > > where we cap the next to INT64_MAX.
> > > > >
> > > > > Couldn't we catch that with this?
> > > > >
> > > > > /* Convert to ns */
> > > > > timecmp_ns = muldiv64(timecmp, NANOSECONDS_PER_SECOND, timebase_freq);
> > > > >
> > > > > if ((NANOSECONDS_PER_SECOND > timebase_freq && timecmp_ns < timecmp)
> > > > >
> > > >
> > > > Yes. But I think there are other problems.
> > > >
> > > > 1. Currently, we are converting the difference between timecmp & rtc_r
> > > > to nanoseconds.
> > > > The diff will always be a smaller number that won't overflow for a long
> > > > time.
> > > > However, if we convert the timestamp directly to nanoseconds, it may
> > > > overflow more often.
> > >
> > > But then we add it back to now right, so it seems pretty similar. This
> > > is ok as is, it just seemed to be more complex then required
> > >
> > > >
> > > > 2. For vstimecmp, we have htimedelta which relies on unsigned integer
> > > > wrapping.
> > > > If we convert directly convert the vstimecmp to nanoseconds, it will
> > > > be problematic to factor in the
> > > > htimedelta.
> > >
> > > Ah ok, I haven't looked at that part yet.
> >
> > Converting htimedelta to nanoseconds is a bigger problem because it is
> > added to time CSR value and typically it is a negative number stored as
> > unsigned long so it's value will be a very high (e.g. 0xFFFFFFxx_yyyyyyyy).
> > Now, if we convert such a large value (at 10MHz) to nanoseconds (at 1GHz)
> > then it will overflow and become a small unsigned long value.
> >
> > To avoid such integer overflow issues, we should convert timer ticks to
> > nanoseconds just before programming the QEMU timer.
> >
> > Regards,
> > Anup
> >
> @Alistair:
> Do we need any more improvement here ? If not I can just rebase and resend.
I think that's it
Alistair