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

Yan Zhao
2024-05-09T03:37:07Z

On Wed, May 08, 2024 at 04:14:24PM -0600, Alex Williamson wrote:
> On Tue, 7 May 2024 17:12:40 +0800
> Yan Zhao <yan.y.zhao@intel.com> wrote:
>
> > On Tue, May 07, 2024 at 04:26:37PM +0800, Tian, Kevin wrote:
> > > > From: Zhao, Yan Y <yan.y.zhao@intel.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,

Thanks, Alex, for pointing it out.
KVM does not mmap this region currently, and I guess KVM will not do the mmap
by itself in future too.

I added this check for VGA range is because I want to call
pat_pfn_immune_to_uc_mtrr() in arch_clean_nonsnoop_dma() in patch 3 to exclude VGA
ranges from CLFLUSH, as arch_clean_nonsnoop_dma() is under arch/x86 and not
virtualization specific.

Also, as Jason once said that "Nothinig about vfio actually guarantees that"
"there's no ISA range" (VGA range), I think KVM might see this range after
hva_to_pfn_remapped() translation, and adding this check may be helpful to KVM,
too.

Thanks
Yan

>
> > > > @@ -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 ||
> > >
> >
>