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

[Chia-I Wu]

On Wed, Feb 19, 2020 at 1:52 AM Tian, Kevin <kevin.tian at intel.com> wrote:
>
> > From: Paolo Bonzini
> > Sent: Wednesday, February 19, 2020 12:29 AM
> >
> > On 14/02/20 23:03, Sean Christopherson wrote:
> > >> On Fri, Feb 14, 2020 at 1:47 PM Chia-I Wu <olvaffe at gmail.com> wrote:
> > >>> AFAICT, it is currently allowed on ARM (verified) and AMD (not
> > >>> verified, but svm_get_mt_mask returns 0 which supposedly means the
> > NPT
> > >>> does not restrict what the guest PAT can do).  This diff would do the
> > >>> trick for Intel without needing any uapi change:
> > >> I would be concerned about Intel CPU errata such as SKX40 and SKX59.
> > > The part KVM cares about, #MC, is already addressed by forcing UC for
> > MMIO.
> > > The data corruption issue is on the guest kernel to correctly use WC
> > > and/or non-temporal writes.
> >
> > What about coherency across live migration?  The userspace process would
> > use cached accesses, and also a WBINVD could potentially corrupt guest
> > memory.
> >
>
> In such case the userspace process possibly should conservatively use
> UC mapping, as if for MMIO regions on a passthrough device. However
> there remains a problem. the definition of KVM_MEM_DMA implies
> favoring guest setting, which could be whatever type in concept. Then
> assuming UC is also problematic. I'm not sure whether inventing another
> interface to query effective memory type from KVM is a good idea. There
> is no guarantee that the guest will use same type for every page in the
> same slot, then such interface might be messy. Alternatively, maybe
> we could just have an interface for KVM userspace to force memory type
> for a given slot, if it is mainly used in para-virtualized scenarios (e.g.
> virtio-gpu) where the guest is enlightened to use a forced type (e.g. WC)?
KVM forcing the memory type for a given slot should work too.  But the
ignore-guest-pat bit seems to be Intel-specific.  We will need to
define how the second-level page attributes combine with the guest
page attributes somehow.

KVM should in theory be able to tell that the userspace region is
mapped with a certain memory type and can force the same memory type
onto the guest.  The userspace does not need to be involved.  But that
sounds very slow?  This may be a dumb question, but would it help to
add KVM_SET_DMA_BUF and let KVM negotiate the memory type with the
in-kernel GPU drivers?


>
> Thanks
> Kevin