On Fri, Feb 14, 2020 at 2:26 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: > > On 13/02/20 23:18, Chia-I Wu wrote: > > > > The bug you mentioned was probably this one > > > > https://bugzilla.kernel.org/show_bug.cgi?id=104091 > > Yes, indeed. > > > From what I can tell, the commit allowed the guests to create cached > > mappings to MMIO regions and caused MCEs. That is different than what > > I need, which is to allow guests to create uncached mappings to system > > ram (i.e., !kvm_is_mmio_pfn) when the host userspace also has uncached > > mappings. But it is true that this still allows the userspace & guest > > kernel to create conflicting memory types. > > Right, the question is whether the MCEs were tied to MMIO regions > specifically and if so why. > > An interesting remark is in the footnote of table 11-7 in the SDM. > There, for the MTRR (EPT for us) memory type UC you can read: > > The UC attribute comes from the MTRRs and the processors are not > required to snoop their caches since the data could never have > been cached. This attribute is preferred for performance reasons. > > There are two possibilities: > > 1) the footnote doesn't apply to UC mode coming from EPT page tables. > That would make your change safe. > > 2) the footnote also applies when the UC attribute comes from the EPT > page tables rather than the MTRRs. In that case, the host should use > UC as the EPT page attribute if and only if it's consistent with the host > MTRRs; it would be more or less impossible to honor UC in the guest MTRRs. > In that case, something like the patch below would be needed. > > It is not clear from the manual why the footnote would not apply to WC; that > is, the manual doesn't say explicitly that the processor does not do snooping > for accesses to WC memory. But I guess that must be the case, which is why I > used MTRR_TYPE_WRCOMB in the patch below. > > Either way, we would have an explanation of why creating cached mapping to > MMIO regions would, and why in practice we're not seeing MCEs for guest RAM > (the guest would have set WB for that memory in its MTRRs, not UC). > > One thing you didn't say: how would userspace use KVM_MEM_DMA? On which > regions would it be set? It will be set for shmems that are mapped WC. GPU/DRM drivers allocate shmems as DMA-able gpu buffers and allow the userspace to map them cached or WC (I915_MMAP_WC or AMDGPU_GEM_CREATE_CPU_GTT_USWC for example). When a shmem is mapped WC and is made available to the guest, we would like the ability to map the region WC in the guest. > Thanks, > > Paolo > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c > index dc331fb06495..2be6f7effa1d 100644 > --- a/arch/x86/kvm/vmx/vmx.c > +++ b/arch/x86/kvm/vmx/vmx.c > @@ -6920,8 +6920,16 @@ static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio) > } > > cache = kvm_mtrr_get_guest_memory_type(vcpu, gfn); > - > exit: > + if (cache == MTRR_TYPE_UNCACHABLE && !is_mmio) { > + /* > + * We cannot set UC in the EPT page tables as it can cause > + * machine check exceptions (??). Hopefully the guest is > + * using PAT. > + */ > + cache = MTRR_TYPE_WRCOMB; > + } > + > return (cache << VMX_EPT_MT_EPTE_SHIFT) | ipat; > } > >
