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

[Chia-I Wu]

On Thu, Feb 13, 2020 at 1:41 PM Paolo Bonzini <pbonzini at redhat.com> wrote:
>
> On 13/02/20 22:30, Chia-I Wu wrote:
> > Hi,
> >
> > Host GPU drivers like to give userspace WC mapping.  When the userspace makes
> > the mapping available to a guest, it also tells the guest to create a WC
> > mapping.  However, even when the guest kernel picks the correct memory type,
> > it gets ignored because of VMX_EPT_IPAT_BIT on Intel.
> >
> > This series adds a new flag to KVM_SET_USER_MEMORY_REGION, which tells the
> > host kernel to honor the guest memory type for the memslot.  An alternative
> > fix is for KVM to unconditionally honor the guest memory type (unless it is
> > MMIO, to avoid MCEs on Intel).  I believe the alternative fix is how things
> > are on ARM, and probably also how things are on AMD.
> >
> > I am new to KVM and HW virtualization technologies.  This series is meant as
> > an RFC.
> >
>
> When we tried to do this in the past, we got machine checks everywhere
> unfortunately due to the same address being mapped with different memory
> types.  Unfortunately I cannot find the entry anymore in bugzilla, but
> this was not fixed as far as I know.
Yeah, I did a bit of history digging here

  https://gitlab.freedesktop.org/virgl/virglrenderer/issues/151#note_372594

The bug you mentioned was probably this one

  https://bugzilla.kernel.org/show_bug.cgi?id=104091

which was caused by

  https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=fd717f11015f673487ffc826e59b2bad69d20fe5

>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.

Implementation-wise, the current implementation uses
kvm_arch_register_noncoherent_dma.  It essentially treats a memslot
with the new flag as a non-coherent vfio device as far as mmu is
concerned.

>
> Paolo
>