Re: [Xen-devel] [PATCH for-4.13] x86/tsc: update vcpu time info on guest TSC adjustments

[Wei Liu <wl@xxxxxxx>]

On Wed, Oct 23, 2019 at 10:57:39AM +0200, Roger Pau Monne wrote:
> If a HVM/PVH guest writes to MSR_IA32_TSC{_ADJUST} and thus changes
> the value of the time stamp counter the vcpu time info must also be
> updated, or the time calculated by the guest using the Xen PV clock
> interface will be skewed.
> 
> Update the vcpu time info when the guest writes to either MSR_IA32_TSC
> or MSR_IA32_TSC_ADJUST. This fixes lockups seen when running the
> pv-shim on AMD hardware, since the shim will aggressively try to keep
> TSCs in sync by periodically writing to MSR_IA32_TSC if the TSC is not
> reliable.
> 
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
> Cc: Jürgen Groß <jgross@xxxxxxxx>
> ---
>  xen/arch/x86/hvm/hvm.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index 9acd359c99..e4c0425330 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -442,6 +442,8 @@ static void hvm_set_guest_tsc_msr(struct vcpu *v, u64 
> guest_tsc)
>  
>      hvm_set_guest_tsc(v, guest_tsc);
>      v->arch.hvm.msr_tsc_adjust += v->arch.hvm.cache_tsc_offset - tsc_offset;
> +    if ( v == current )
> +        update_vcpu_system_time(v);

Why not get rid of the test and always call update_vcpu_system_time(v)?

Wei.

>  }
>  
>  static void hvm_set_guest_tsc_adjust(struct vcpu *v, u64 tsc_adjust)
> @@ -449,6 +451,8 @@ static void hvm_set_guest_tsc_adjust(struct vcpu *v, u64 
> tsc_adjust)
>      v->arch.hvm.cache_tsc_offset += tsc_adjust - v->arch.hvm.msr_tsc_adjust;
>      hvm_set_tsc_offset(v, v->arch.hvm.cache_tsc_offset, 0);
>      v->arch.hvm.msr_tsc_adjust = tsc_adjust;
> +    if ( v == current )
> +        update_vcpu_system_time(v);
>  }
>  
>  u64 hvm_get_guest_tsc_fixed(struct vcpu *v, uint64_t at_tsc)
> -- 
> 2.23.0
> 

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