RE: [PATCH 2/5] KVM: x86/mmu: Fine-grained check of whether a invalid & RAM PFN is MMIO

[Tian, Kevin]

> From: Zhao, Yan Y <yan.y.zhao@xxxxxxxxx>
> Sent: Tuesday, May 7, 2024 2:20 PM
> @@ -101,9 +101,21 @@ static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>  			 */
>  			(!pat_enabled() ||
> pat_pfn_immune_to_uc_mtrr(pfn));
> 
> +	/*
> +	 * If the PFN is invalid and not RAM in raw e820 table, keep treating it
> +	 * as MMIO.
> +	 *
> +	 * If the PFN is invalid and is RAM in raw e820 table,
> +	 * - if PAT is not enabled, always treat the PFN as MMIO to avoid
> futher
> +	 *   checking of MTRRs.
> +	 * - if PAT is enabled, treat the PFN as MMIO if its PAT is UC/WC/UC-
> in
> +	 *   primary MMU.
> +	 * to prevent guest cacheable access to MMIO PFNs.
> +	 */
>  	return !e820__mapped_raw_any(pfn_to_hpa(pfn),
>  				     pfn_to_hpa(pfn + 1) - 1,
> -				     E820_TYPE_RAM);
> +				     E820_TYPE_RAM) ? true :
> +				     (!pat_enabled() ||
> pat_pfn_immune_to_uc_mtrr(pfn));

Is it for another theoretical problem in case the primary
mmu uses a non-WB type on a invalid RAM-type pfn so
you want to do additional scrutiny here?