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Re: [Qemu-devel] [PATCH] kvmclock: Add comment explaining why we need cp
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH] kvmclock: Add comment explaining why we need cpu_clean_all_dirty() |
Date: |
Wed, 05 Nov 2014 11:27:34 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 |
On 03/11/2014 18:45, Eduardo Habkost wrote:
> Try to explain why commit 317b0a6d8ba44e9bf8f9c3dbd776c4536843d82c
> needed a cpu_clean_all_dirty() call just after calling
> cpu_synchronize_all_states().
>
> Signed-off-by: Eduardo Habkost <address@hidden>
> Cc: Andrey Korolyov <address@hidden>
> Cc: Marcin GibuĊa <address@hidden>
> Cc: Marcelo Tosatti <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> ---
> hw/i386/kvm/clock.c | 14 ++++++++++++++
> 1 file changed, 14 insertions(+)
>
> diff --git a/hw/i386/kvm/clock.c b/hw/i386/kvm/clock.c
> index 1ac60d6..58be2bd 100644
> --- a/hw/i386/kvm/clock.c
> +++ b/hw/i386/kvm/clock.c
> @@ -127,7 +127,21 @@ static void kvmclock_vm_state_change(void *opaque, int
> running,
> }
>
> cpu_synchronize_all_states();
> + /* In theory, the cpu_synchronize_all_states() call above wouldn't
> + * affect the rest of the code, as the VCPU state inside CPUState
> + * is supposed to always match the VCPU state on the kernel side.
> + *
> + * In practice, calling cpu_synchronize_state() too soon will load
> the
> + * kernel-side APIC state into X86CPU.apic_state too early, APIC
> state
> + * won't be reloaded later because CPUState.vcpu_dirty==true, and
> + * outdated APIC state may be migrated to another host.
> + *
> + * The real fix would be to make sure outdated APIC state is read
> + * from the kernel again when necessary. While this is not fixed, we
> + * need the cpu_clean_all_dirty() call below.
> + */
> cpu_clean_all_dirty();
> +
> ret = kvm_vm_ioctl(kvm_state, KVM_GET_CLOCK, &data);
> if (ret < 0) {
> fprintf(stderr, "KVM_GET_CLOCK failed: %s\n", strerror(ret));
>
Thanks, applying to uq/master.
Paolo