Re: [PATCH v3 5/6] KVM: x86: remove shadow_memtype_mask

[Yan Zhao <yan.y.zhao@intel.com>]

On Tue, Mar 04, 2025 at 01:06:46AM -0500, Paolo Bonzini wrote:
> The IGNORE_GUEST_PAT quirk is inapplicable, and thus always-disabled,
> if shadow_memtype_mask is zero.  As long as vmx_get_mt_mask is not
For shadow paging case, current KVM always ignores guest PAT, i.e., the quirk is
always-enabled.

However, this might be negligible, as non-coherent DMA is unlikely to function
well with shadow paging anyway, if I don't miss anything.

> called for the shadow paging case, there is no need to consult
> shadow_memtype_mask and it can be removed altogether.
... 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 5b45fca3ddfa..8bf50cecc75c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -13544,8 +13544,10 @@ static void kvm_noncoherent_dma_assignment_start_or_stop(struct kvm *kvm)
>  	 * due to toggling the "ignore PAT" bit.  Zap all SPTEs when the first
>  	 * (or last) non-coherent device is (un)registered to so that new SPTEs
>  	 * with the correct "ignore guest PAT" setting are created.
> +	 *
> +	 * If KVM always honors guest PAT, however, there is nothing to do.
>  	 */
> -	if (kvm_mmu_may_ignore_guest_pat(kvm))
> +	if (kvm_check_has_quirk(kvm, KVM_X86_QUIRK_IGNORE_GUEST_PAT))
>  		kvm_zap_gfn_range(kvm, gpa_to_gfn(0), gpa_to_gfn(~0ULL));
>  }
>  
> -- 
> 2.43.5
> 
>