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, 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 || > > >