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: Wed, 8 May 2024 16:14:24 -0600
From: Alex Williamson <alex.williamson@...hat.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, "kvm@...r.kernel.org"
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Subject: Re: [PATCH 1/5] x86/pat: Let pat_pfn_immune_to_uc_mtrr() check MTRR
 for untracked PAT range

On Tue, 7 May 2024 17:12:40 +0800
Yan Zhao <yan.y.zhao@...el.com> wrote:

> 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.

include/uapi/linux/vfio.h:
        /*
         * Expose VGA regions defined for PCI base class 03, subclass 00.
         * This includes I/O port ranges 0x3b0 to 0x3bb and 0x3c0 to 0x3df
         * as well as the MMIO range 0xa0000 to 0xbffff.  Each implemented
         * range is found at it's identity mapped offset from the region
         * offset, for example 0x3b0 is region_info.offset + 0x3b0.  Areas
         * between described ranges are unimplemented.
         */
        VFIO_PCI_VGA_REGION_INDEX,

We don't currently support mmap for this region though, so I think we
still don't technically require this, but I guess an mmap through KVM
is theoretically possible.  Thanks,

Alex

> > > @@ -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 ||  
> >   
>