> From: Chia-I Wu > Sent: Saturday, February 15, 2020 5:15 AM > > 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. Curious... How is such slot exposed to the guest? A reserved memory region? Is it static or might be dynamically added? Thanks Kevin
