Re: [Qemu-devel] [RFC v3 03/13] softmmu: Add helpers for a new slow-path

[Alex Bennée]

Alvise Rigo <address@hidden> writes:

> The new helpers rely on the legacy ones to perform the actual read/write.
>
> The StoreConditional helper (helper_le_stcond_name) returns 1 if the
> store has to fail due to a concurrent access to the same page by
> another vCPU.  A 'concurrent access' can be a store made by *any* vCPU
> (although, some implementations allow stores made by the CPU that issued
> the LoadLink).
>
> These helpers also update the TLB entry of the page involved in the
> LL/SC, so that all the following accesses made by any vCPU will follow
> the slow path.
> In real multi-threading, these helpers will require to temporarily pause
> the execution of the other vCPUs in order to update accordingly (flush)
> the TLB cache.
>
> Suggested-by: Jani Kokkonen <address@hidden>
> Suggested-by: Claudio Fontana <address@hidden>
> Signed-off-by: Alvise Rigo <address@hidden>
> ---
>  cputlb.c                |   3 +
>  softmmu_llsc_template.h | 155 
> ++++++++++++++++++++++++++++++++++++++++++++++++
>  softmmu_template.h      |   4 ++
>  tcg/tcg.h               |  18 ++++++
>  4 files changed, 180 insertions(+)
>  create mode 100644 softmmu_llsc_template.h
>
> diff --git a/cputlb.c b/cputlb.c
> index 0aca407..fa38714 100644
> --- a/cputlb.c
> +++ b/cputlb.c
> @@ -475,6 +475,8 @@ tb_page_addr_t get_page_addr_code(CPUArchState *env1, 
> target_ulong addr)
>  
>  #define MMUSUFFIX _mmu
>  
> +/* Generates LoadLink/StoreConditional helpers in softmmu_template.h */
> +#define GEN_EXCLUSIVE_HELPERS
>  #define SHIFT 0
>  #include "softmmu_template.h"
>  
> @@ -487,6 +489,7 @@ tb_page_addr_t get_page_addr_code(CPUArchState *env1, 
> target_ulong addr)
>  #define SHIFT 3
>  #include "softmmu_template.h"
>  #undef MMUSUFFIX
> +#undef GEN_EXCLUSIVE_HELPERS
>  
>  #define MMUSUFFIX _cmmu
>  #undef GETPC_ADJ
> diff --git a/softmmu_llsc_template.h b/softmmu_llsc_template.h
> new file mode 100644
> index 0000000..81e9d8e
> --- /dev/null
> +++ b/softmmu_llsc_template.h
> @@ -0,0 +1,155 @@
> +/*
> + *  Software MMU support (esclusive load/store operations)
> + *
> + * Generate helpers used by TCG for qemu_ldlink/stcond ops.
> + *
> + * Included from softmmu_template.h only.
> + *
> + * Copyright (c) 2015 Virtual Open Systems
> + *
> + * Authors:
> + *  Alvise Rigo <address@hidden>
> + *
> + * This library is free software; you can redistribute it and/or
> + * modify it under the terms of the GNU Lesser General Public
> + * License as published by the Free Software Foundation; either
> + * version 2 of the License, or (at your option) any later version.
> + *
> + * This library is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> + * Lesser General Public License for more details.
> + *
> + * You should have received a copy of the GNU Lesser General Public
> + * License along with this library; if not, see 
> <http://www.gnu.org/licenses/>.
> + */
> +
> +/*
> + * TODO configurations not implemented:
> + *     - Signed/Unsigned Big-endian
> + *     - Signed Little-endian
> + * */
> +
> +#if DATA_SIZE > 1
> +#define helper_le_ldlink_name  glue(glue(helper_le_ldlink, USUFFIX), 
> MMUSUFFIX)
> +#define helper_le_stcond_name  glue(glue(helper_le_stcond, SUFFIX), 
> MMUSUFFIX)
> +#else
> +#define helper_le_ldlink_name  glue(glue(helper_ret_ldlink, USUFFIX), 
> MMUSUFFIX)
> +#define helper_le_stcond_name  glue(glue(helper_ret_stcond, SUFFIX), 
> MMUSUFFIX)
> +#endif
> +
> +/* helpers from cpu_ldst.h, byte-order independent versions */
> +#if DATA_SIZE > 1
> +#define helper_ld_legacy glue(glue(helper_le_ld, USUFFIX), MMUSUFFIX)
> +#define helper_st_legacy glue(glue(helper_le_st, SUFFIX), MMUSUFFIX)
> +#else
> +#define helper_ld_legacy glue(glue(helper_ret_ld, USUFFIX), MMUSUFFIX)
> +#define helper_st_legacy glue(glue(helper_ret_st, SUFFIX), MMUSUFFIX)
> +#endif
> +
> +#define is_write_tlb_entry_set(env, page, index)                             
> \
> +({                                                                           
> \
> +    (addr & TARGET_PAGE_MASK)                                                
> \
> +         == ((env->tlb_table[mmu_idx][index].addr_write) &                   
> \
> +                 (TARGET_PAGE_MASK | TLB_INVALID_MASK));                     
> \
> +})                                                                           
> \
> +
> +#define EXCLUSIVE_RESET_ADDR ULLONG_MAX
> +
> +WORD_TYPE helper_le_ldlink_name(CPUArchState *env, target_ulong addr,
> +                                TCGMemOpIdx oi, uintptr_t retaddr)
> +{
> +    WORD_TYPE ret;
> +    int index;
> +    CPUState *cpu;
> +    hwaddr hw_addr;
> +    unsigned mmu_idx = get_mmuidx(oi);
> +
> +    /* Use the proper load helper from cpu_ldst.h */
> +    ret = helper_ld_legacy(env, addr, mmu_idx, retaddr);
> +
> +    /* The last legacy access ensures that the TLB and IOTLB entry for 'addr'
> +     * have been created. */
> +    index = (addr >> TARGET_PAGE_BITS) & (CPU_TLB_SIZE - 1);
> +
> +    /* hw_addr = hwaddr of the page (i.e. section->mr->ram_addr + xlat)
> +     * plus the offset (i.e. addr & ~TARGET_PAGE_MASK) */
> +    hw_addr = (env->iotlb[mmu_idx][index].addr & TARGET_PAGE_MASK) +
>  addr;

I'm having trouble parsing the comment w.r.t the code here. Aren't the
TLB entries already page aligned? Should you not have:

    index = (addr >> TARGET_PAGE_BITS) & (CPU_TLB_SIZE - 1);
    offset = (addr & ~TARGET_PAGE_MASK)

    hw_addr = env->iotlb[mmu_idx][index].addr + offset

?

> +
> +    /* Set the exclusive-protected hwaddr. */
> +    env->excl_protected_hwaddr = hw_addr;
> +    env->ll_sc_context = true;
> +
> +    /* No need to mask hw_addr with TARGET_PAGE_MASK since
> +     * cpu_physical_memory_excl_is_dirty() will take care of that. */
> +    if (cpu_physical_memory_excl_is_dirty(hw_addr)) {
> +        cpu_physical_memory_clear_excl_dirty(hw_addr);
> +
> +        /* Invalidate the TLB entry for the other processors. The next TLB
> +         * entries for this page will have the TLB_EXCL flag set. */
> +        CPU_FOREACH(cpu) {
> +            if (cpu != current_cpu) {
> +                tlb_flush(cpu, 1);
> +            }
> +        }
> +    }
> +
> +    /* For this vCPU, just update the TLB entry, no need to flush. */
> +    env->tlb_table[mmu_idx][index].addr_write |= TLB_EXCL;
> +
> +    return ret;
> +}
> +
> +WORD_TYPE helper_le_stcond_name(CPUArchState *env, target_ulong addr,
> +                                DATA_TYPE val, TCGMemOpIdx oi,
> +                                uintptr_t retaddr)
> +{
> +    WORD_TYPE ret;
> +    int index;
> +    hwaddr hw_addr;
> +    unsigned mmu_idx = get_mmuidx(oi);
> +
> +    /* If the TLB entry is not the right one, create it. */
> +    index = (addr >> TARGET_PAGE_BITS) & (CPU_TLB_SIZE - 1);
> +    if (!is_write_tlb_entry_set(env, addr, index)) {
> +        tlb_fill(ENV_GET_CPU(env), addr, MMU_DATA_STORE, mmu_idx, retaddr);
> +    }
> +
> +    /* hw_addr = hwaddr of the page (i.e. section->mr->ram_addr + xlat)
> +     * plus the offset (i.e. addr & ~TARGET_PAGE_MASK) */
> +    hw_addr = (env->iotlb[mmu_idx][index].addr & TARGET_PAGE_MASK) + addr;
> +
> +    if (!env->ll_sc_context) {
> +        /* No LoakLink has been set, the StoreCond has to fail. */
> +        return 1;
> +    }
> +
> +    env->ll_sc_context = 0;
> +
> +    if (cpu_physical_memory_excl_is_dirty(hw_addr)) {
> +        /* Another vCPU has accessed the memory after the LoadLink. */
> +        ret = 1;
> +    } else {
> +        helper_st_legacy(env, addr, val, mmu_idx, retaddr);
> +
> +        /* The StoreConditional succeeded */
> +        ret = 0;
> +    }
> +
> +    env->tlb_table[mmu_idx][index].addr_write &= ~TLB_EXCL;
> +    env->excl_protected_hwaddr = EXCLUSIVE_RESET_ADDR;
> +    /* It's likely that the page will be used again for exclusive accesses,
> +     * for this reason we don't flush any TLB cache at the price of some
> +     * additional slow paths and we don't set the page bit as dirty.
> +     * The EXCL TLB entries will not remain there forever since they will be
> +     * eventually removed to serve another guest page; when this happens we
> +     * remove also the dirty bit (see cputlb.c).
> +     * */

We are explaining code that was never added here in the first place?

> +
> +    return ret;
> +}
> +
> +#undef helper_le_ldlink_name
> +#undef helper_le_stcond_name
> +#undef helper_ld_legacy
> +#undef helper_st_legacy
> diff --git a/softmmu_template.h b/softmmu_template.h
> index 0edd451..bc767f6 100644
> --- a/softmmu_template.h
> +++ b/softmmu_template.h
> @@ -630,6 +630,10 @@ void probe_write(CPUArchState *env, target_ulong addr, 
> int mmu_idx,
>  #endif
>  #endif /* !defined(SOFTMMU_CODE_ACCESS) */
>  
> +#ifdef GEN_EXCLUSIVE_HELPERS
> +#include "softmmu_llsc_template.h"
> +#endif
> +
>  #undef READ_ACCESS_TYPE
>  #undef SHIFT
>  #undef DATA_TYPE
> diff --git a/tcg/tcg.h b/tcg/tcg.h
> index 032fe10..8ca85ab 100644
> --- a/tcg/tcg.h
> +++ b/tcg/tcg.h
> @@ -962,6 +962,15 @@ tcg_target_ulong helper_be_ldul_mmu(CPUArchState *env, 
> target_ulong addr,
>                                      TCGMemOpIdx oi, uintptr_t retaddr);
>  uint64_t helper_be_ldq_mmu(CPUArchState *env, target_ulong addr,
>                             TCGMemOpIdx oi, uintptr_t retaddr);
> +/* Exclusive variants */
> +tcg_target_ulong helper_ret_ldlinkub_mmu(CPUArchState *env, target_ulong 
> addr,
> +                                         int mmu_idx, uintptr_t retaddr);
> +tcg_target_ulong helper_le_ldlinkuw_mmu(CPUArchState *env, target_ulong addr,
> +                                        int mmu_idx, uintptr_t retaddr);
> +tcg_target_ulong helper_le_ldlinkul_mmu(CPUArchState *env, target_ulong addr,
> +                                        int mmu_idx, uintptr_t retaddr);
> +uint64_t helper_le_ldlinkq_mmu(CPUArchState *env, target_ulong addr,
> +                               int mmu_idx, uintptr_t retaddr);
>  
>  /* Value sign-extended to tcg register size.  */
>  tcg_target_ulong helper_ret_ldsb_mmu(CPUArchState *env, target_ulong addr,
> @@ -989,6 +998,15 @@ void helper_be_stl_mmu(CPUArchState *env, target_ulong 
> addr, uint32_t val,
>                         TCGMemOpIdx oi, uintptr_t retaddr);
>  void helper_be_stq_mmu(CPUArchState *env, target_ulong addr, uint64_t val,
>                         TCGMemOpIdx oi, uintptr_t retaddr);
> +/* Exclusive variants */
> +tcg_target_ulong helper_ret_stcondb_mmu(CPUArchState *env, target_ulong addr,
> +                                uint8_t val, int mmu_idx, uintptr_t retaddr);
> +tcg_target_ulong helper_le_stcondw_mmu(CPUArchState *env, target_ulong addr,
> +                                uint16_t val, int mmu_idx, uintptr_t 
> retaddr);
> +tcg_target_ulong helper_le_stcondl_mmu(CPUArchState *env, target_ulong addr,
> +                                uint32_t val, int mmu_idx, uintptr_t 
> retaddr);
> +uint64_t helper_le_stcondq_mmu(CPUArchState *env, target_ulong addr,
> +                                uint64_t val, int mmu_idx, uintptr_t 
> retaddr);
>  
>  /* Temporary aliases until backends are converted.  */
>  #ifdef TARGET_WORDS_BIGENDIAN

-- 
Alex Bennée