Re: [PATCH v2] KVM: x86: Avoid guest page table walk when gpa_available is set

[Brijesh Singh]

Hi Jorge,

On 05/30/2017 04:05 AM, Joerg Roedel wrote:
> On Fri, May 19, 2017 at 11:12:29AM -0400, Brijesh Singh wrote:
> > From: Brijesh Singh <brijesh.singh@xxxxxxx>
> >
> > On AMD hardware when a guest causes a NPF which requires emulation,
> > the vcpu->arch.gpa_available flag is set to indicate that cr2 contains
> > a valid GPA.
> >
> > Currently, emulator_read_write_onepage() makes use of gpa_available flag
> > to avoid a guest page walk for a known MMIO regions. Lets not limit
> > the gpa_available optimization to just MMIO region. The patch extends
> > the check to avoid page walk whenever gpa_available flag is set.
> >
> > Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> > ---
> > v1: http://marc.info/?l=kvm&m=149304930814202&w=2
> >
> > Changes in v2:
> >   - move gpa_val setting in pf_interception
> >
> >   arch/x86/include/asm/kvm_host.h |  1 +
> >   arch/x86/kvm/svm.c              |  4 ++++
> >   arch/x86/kvm/x86.c              | 14 +++++++-------
> >   3 files changed, 12 insertions(+), 7 deletions(-)
>
> I havn't checked in detail, but maybe you have: Does that take nesting
> into account, where we might run on a shadow nested page-table and we
> actually get a nested gpa (which still needs translation) instead of a
> normal gpa?

nesting should not be an issue. all we're doing is avoiding the GVA->GPA
translation for the lowest level guest. The nested case should still use
the original code path and perform the nested page walk. I have verified
the nesting support and it all seem to work just fine. You can find the
original gpa_available patch here [1]

[1] https://git.kernel.org/pub/scm/virt/kvm/kvm.git/commit/?id=0f89b207b04a1a399e19d35293658e3a571da3d7

-Brijesh