RE: [Xen-devel] [PATCH] No need to emulate WBINVD when VT-d force snooping feature available

[Ian Pratt <Ian.Pratt@xxxxxxxxxxxxx>]

> There is no cache coherency issue if VT-d engine's force snooping feature
> available.

Isn't it also necessary to explicitly set the SNP bit in each VTd PTE to force 
snooping? I don't believe we do this today, but since we ignore guest PAT 
settings on RAM we should do so.

Thanks,
Ian

 
> Signed-off-by: Sheng Yang <sheng@xxxxxxxxxxxxxxx>
> 
> diff --git a/xen/arch/x86/hvm/vmx/vmcs.c b/xen/arch/x86/hvm/vmx/vmcs.c
> --- a/xen/arch/x86/hvm/vmx/vmcs.c
> +++ b/xen/arch/x86/hvm/vmx/vmcs.c
> @@ -1064,8 +1064,10 @@
>           *  1: flushing cache (wbinvd) when the guest is scheduled out if
>           *     there is no wbinvd exit, or
>           *  2: execute wbinvd on all dirty pCPUs when guest wbinvd exits.
> +         * If VT-d engine can force snooping, we don't need to do these.
>           */
> -        if ( has_arch_pdevs(v->domain) && !cpu_has_wbinvd_exiting )
> +        if ( has_arch_pdevs(v->domain) && !iommu_snoop
> +                && !cpu_has_wbinvd_exiting )
>          {
>              int cpu = v->arch.hvm_vmx.active_cpu;
>              if ( cpu != -1 )
> diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
> --- a/xen/arch/x86/hvm/vmx/vmx.c
> +++ b/xen/arch/x86/hvm/vmx/vmx.c
> @@ -2089,6 +2089,9 @@
>      if ( !has_arch_mmios(current->domain) )
>          return;
> 
> +    if ( iommu_snoop )
> +        return;
> +
>      if ( cpu_has_wbinvd_exiting )
>          on_each_cpu(wbinvd_ipi, NULL, 1);
>      else

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