[RFC PATCH 0/3] KVM: x86: honor guest memory type

[Tian, Kevin]

> From: Chia-I Wu
> Sent: Saturday, February 15, 2020 5:15 AM
> 
> On Fri, Feb 14, 2020 at 2:26 AM Paolo Bonzini <pbonzini at redhat.com> 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