linux-kernel - Re: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR for untracked PAT range

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Date: Tue, 7 May 2024 17:12:40 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
CC: "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>, "alex.williamson@...hat.com"
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	"robin.murphy@....com" <robin.murphy@....com>, "baolu.lu@...ux.intel.com"
	<baolu.lu@...ux.intel.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR
 for untracked PAT range

On Tue, May 07, 2024 at 04:26:37PM +0800, Tian, Kevin wrote:
> > From: Zhao, Yan Y <yan.y.zhao@...el.com>
> > 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?

Yes. Not sure if VGA range is allowed to be exposed to guest in future, given
we have VFIO variant drivers.

> 
> > @@ -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.
Right.
Do you think we can rename this function to something like
pfn_of_uncachable_effective_memory_type() and make it work under !pat_enabled()
too?

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