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

[Chia-I Wu <olvaffe@gmail.com>]

On Wed, Feb 19, 2020 at 2:00 AM Tian, Kevin <kevin.tian@intel.com> wrote:
>
> > From: Chia-I Wu
> > Sent: Saturday, February 15, 2020 5:15 AM
> >
> > On Fri, Feb 14, 2020 at 2:26 AM Paolo Bonzini <pbonzini@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?
The plan is for virtio-gpu device to reserve a huge memory region in
the guest.  Memslots may be added dynamically or statically to back
the region.

Dynamic: the host adds a 16MB GPU allocation as a memslot at a time.
The guest kernel suballocates from the 16MB pool.

Static: the host creates a huge PROT_NONE memfd and adds it as a
memslot.  GPU allocations are mremap()ed into the memfd region to
provide the real mapping.

These options are considered because the number of memslots are
limited: 32 on ARM and 509 on x86.  If the number of memslots could be
made larger (4096 or more), we would also consider adding each
individual GPU allocation as a memslot.

These are actually questions we need feedback.  Besides, GPU
allocations can be assumed to be kernel dma-bufs in this context.  I
wonder if it makes sense to have a variation of
KVM_SET_USER_MEMORY_REGION that takes dma-bufs.


>
> Thanks
> Kevin
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