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Re: [RFC PATCH 09/10] target/riscv: Restrict KVM-specific fields from Ar

From: Daniel Henrique Barboza
Subject: Re: [RFC PATCH 09/10] target/riscv: Restrict KVM-specific fields from ArchCPU
Date: Wed, 5 Apr 2023 17:44:34 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.0

On 4/5/23 13:04, Philippe Mathieu-Daudé wrote:
These fields shouldn't be accessed when KVM is not available.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
RFC: The migration part is likely invalid...

kvmtimer_needed() is defined in target/riscv/machine.c as

   static bool kvmtimer_needed(void *opaque)
       return kvm_enabled();

which depends on a host feature.

kvm_enabled() can be false even when CONFIG_KVM is true when a KVM capable host
is running a TCG guest, for example. In that case env->kvm_timer_* states exist
but aren't initialized, and shouldn't be migrated.

Thus it's not just a host feature, but a host feature + accel option. I think
this is fine.

  target/riscv/cpu.h     | 2 ++
  target/riscv/machine.c | 4 ++++
  2 files changed, 6 insertions(+)

diff --git a/target/riscv/cpu.h b/target/riscv/cpu.h
index 638e47c75a..82939235ab 100644
--- a/target/riscv/cpu.h
+++ b/target/riscv/cpu.h
@@ -377,12 +377,14 @@ struct CPUArchState {
      hwaddr kernel_addr;
      hwaddr fdt_addr;
+#ifdef CONFIG_KVM
      /* kvm timer */
      bool kvm_timer_dirty;
      uint64_t kvm_timer_time;
      uint64_t kvm_timer_compare;
      uint64_t kvm_timer_state;
      uint64_t kvm_timer_frequency;
+#endif /* CONFIG_KVM */
diff --git a/target/riscv/machine.c b/target/riscv/machine.c
index 9c455931d8..e45d564ec3 100644
--- a/target/riscv/machine.c
+++ b/target/riscv/machine.c
@@ -201,10 +201,12 @@ static bool kvmtimer_needed(void *opaque)
static int cpu_post_load(void *opaque, int version_id)
+#ifdef CONFIG_KVM
      RISCVCPU *cpu = opaque;
      CPURISCVState *env = &cpu->env;
env->kvm_timer_dirty = true;
      return 0;
@@ -215,9 +217,11 @@ static const VMStateDescription vmstate_kvmtimer = {
      .needed = kvmtimer_needed,
      .post_load = cpu_post_load,
      .fields = (VMStateField[]) {
+#ifdef CONFIG_KVM
          VMSTATE_UINT64(env.kvm_timer_time, RISCVCPU),
          VMSTATE_UINT64(env.kvm_timer_compare, RISCVCPU),
          VMSTATE_UINT64(env.kvm_timer_state, RISCVCPU),

Here you're creating an empty 'cpu/kvmtimer' vmstate that won't be migrated 
because kvmtimer_needed (== kvm_enabled()) will be always false if CONFIG_KVM=n.

I'd say it's better to just get rid of the whole vmstate in this case, but I 
like the precedence of having vmstates being gated by build flags.

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>


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