Re: [PATCH v3 3/4] kvm: vmx: Add last_cpu to struct vcpu_vmx

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Jun 02, 2020 at 10:33:51AM -0700, Jim Mattson wrote:
> On Mon, Jun 1, 2020 at 6:21 PM Sean Christopherson
> <sean.j.christopherson@intel.com> wrote:
> >
> > On Mon, Jun 01, 2020 at 03:24:15PM -0700, Jim Mattson wrote:
> > > As we already do in svm, record the last logical processor on which a
> > > vCPU has run, so that it can be communicated to userspace for
> > > potential hardware errors.
> > >
> > > Signed-off-by: Jim Mattson <jmattson@google.com>
> > > Reviewed-by: Oliver Upton <oupton@google.com>
> > > Reviewed-by: Peter Shier <pshier@google.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmx.c | 1 +
> > >  arch/x86/kvm/vmx/vmx.h | 3 +++
> > >  2 files changed, 4 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > > index 170cc76a581f..42856970d3b8 100644
> > > --- a/arch/x86/kvm/vmx/vmx.c
> > > +++ b/arch/x86/kvm/vmx/vmx.c
> > > @@ -6730,6 +6730,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
> > >       if (vcpu->arch.cr2 != read_cr2())
> > >               write_cr2(vcpu->arch.cr2);
> > >
> > > +     vmx->last_cpu = vcpu->cpu;
> >
> > This is redundant in the EXIT_FASTPATH_REENTER_GUEST case.  Setting it
> > before reenter_guest is technically wrong if emulation_required is true, but
> > that doesn't seem like it'd be an issue in practice.
> 
> I really would like to capture the last logical processor to execute
> VMLAUNCH/VMRESUME (or VMRUN on the AMD side) on behalf of this vCPU.

Does it matter though?  The flows that consume the variable are all directly
in the VM-Exit path.

> > >       vmx->fail = __vmx_vcpu_run(vmx, (unsigned long *)&vcpu->arch.regs,
> > >                                  vmx->loaded_vmcs->launched);
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> > > index 672c28f17e49..8a1e833cf4fb 100644
> > > --- a/arch/x86/kvm/vmx/vmx.h
> > > +++ b/arch/x86/kvm/vmx/vmx.h
> > > @@ -302,6 +302,9 @@ struct vcpu_vmx {
> > >       u64 ept_pointer;
> > >
> > >       struct pt_desc pt_desc;
> > > +
> > > +     /* which host CPU was used for running this vcpu */
> > > +     unsigned int last_cpu;
> >
> > Why not put this in struct kvm_vcpu_arch?  I'd also vote to name it
> > last_run_cpu, as last_cpu is super misleading.
> 
> I think last_run_cpu may also be misleading, since in the cases of
> interest, nothing actually 'ran.' Maybe last_attempted_vmentry_cpu?

Ya, that thought crossed my mind as well.

> > And if it's in arch, what about setting it vcpu_enter_guest?
> 
> As you point out above, this isn't entirely accurate. (But that's the
> way we roll in kvm, isn't it? :-)

As an alternative to storing the last run/attempted CPU, what about moving
the "bad VM-Exit" detection into handle_exit_irqoff, or maybe a new hook
that is called after IRQs are enabled but before preemption is enabled, e.g.
detect_bad_exit or something?  All of the paths in patch 4/4 can easily be
moved out of handle_exit.  VMX would require a little bit of refacotring for
it's "no handler" check, but that should be minor.