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@xxxxxxxxx> wrote: > > > On Tue, May 07, 2024 at 04:26:37PM +0800, Tian, Kevin wrote: > > > > 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? > > > > 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 || > > > > > >