RE: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR for untracked PAT range

[Tian, Kevin]

> From: Zhao, Yan Y <yan.y.zhao@xxxxxxxxx>
> Sent: Tuesday, May 7, 2024 2:19 PM
> 
> However, lookup_memtype() defaults to returning WB for PFNs within the
> untracked PAT range, regardless of their actual MTRR type. This behavior
> could lead KVM to misclassify the PFN as non-MMIO, permitting cacheable
> guest access. Such access might result in MCE on certain platforms, (e.g.
> clflush on VGA range (0xA0000-0xBFFFF) triggers MCE on some platforms).

the VGA range is not exposed to any guest today. So is it just trying to
fix a theoretical problem?

> @@ -705,7 +705,17 @@ static enum page_cache_mode
> lookup_memtype(u64 paddr)
>   */
>  bool pat_pfn_immune_to_uc_mtrr(unsigned long pfn)
>  {
> -	enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
> +	u64 paddr = PFN_PHYS(pfn);
> +	enum page_cache_mode cm;
> +
> +	/*
> +	 * Check MTRR type for untracked pat range since lookup_memtype()
> always
> +	 * returns WB for this range.
> +	 */
> +	if (x86_platform.is_untracked_pat_range(paddr, paddr + PAGE_SIZE))
> +		cm = pat_x_mtrr_type(paddr, paddr + PAGE_SIZE,
> _PAGE_CACHE_MODE_WB);

doing so violates the name of this function. The PAT of the untracked
range is still WB and not immune to UC MTRR.

> +	else
> +		cm = lookup_memtype(paddr);
> 
>  	return cm == _PAGE_CACHE_MODE_UC ||
>  	       cm == _PAGE_CACHE_MODE_UC_MINUS ||