Re: [PATCH 07/12] KVM: nVMX: Disable vmcs02 posted interrupts if vmcs12 PID isn't mappable

From: Sean Christopherson <seanjc@google.com>
To: Jim Mattson <jmattson@google.com>
Cc: kvm list <kvm@vger.kernel.org>, Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 07/12] KVM: nVMX: Disable vmcs02 posted interrupts if vmcs12 PID isn't mappable
Date: Mon, 24 May 2021 23:45:30 +0000	[thread overview]
Message-ID: <YKw6mpWe3UFY2Gnp@google.com> (raw)
In-Reply-To: <CALMp9eQikiZRzX+UtdTW4rHO+jT2uo5xmrtGGs1V96kEZAHh_A@mail.gmail.com>

On Mon, May 24, 2021, Jim Mattson wrote:
> On Mon, May 24, 2021 at 4:22 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, May 20, 2021, Jim Mattson wrote:
> > > Don't allow posted interrupts to modify a stale posted interrupt
> > > descriptor (including the initial value of 0).
> > >
> > > Empirical tests on real hardware reveal that a posted interrupt
> > > descriptor referencing an unbacked address has PCI bus error semantics
> > > (reads as all 1's; writes are ignored). However, kvm can't distinguish
> > > unbacked addresses from device-backed (MMIO) addresses, so it should
> > > really ask userspace for an MMIO completion. That's overly
> > > complicated, so just punt with KVM_INTERNAL_ERROR.
> > >
> > > Don't return the error until the posted interrupt descriptor is
> > > actually accessed. We don't want to break the existing kvm-unit-tests
> > > that assume they can launch an L2 VM with a posted interrupt
> > > descriptor that references MMIO space in L1.
> > >
> > > Fixes: 6beb7bd52e48 ("kvm: nVMX: Refactor nested_get_vmcs12_pages()")
> > > Signed-off-by: Jim Mattson <jmattson@google.com>
> > > ---
> > >  arch/x86/kvm/vmx/nested.c | 15 ++++++++++++++-
> > >  1 file changed, 14 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > index 706c31821362..defd42201bb4 100644
> > > --- a/arch/x86/kvm/vmx/nested.c
> > > +++ b/arch/x86/kvm/vmx/nested.c
> > > @@ -3175,6 +3175,15 @@ static bool nested_get_vmcs12_pages(struct kvm_vcpu *vcpu)
> > >                               offset_in_page(vmcs12->posted_intr_desc_addr));
> > >                       vmcs_write64(POSTED_INTR_DESC_ADDR,
> > >                                    pfn_to_hpa(map->pfn) + offset_in_page(vmcs12->posted_intr_desc_addr));
> > > +             } else {
> > > +                     /*
> > > +                      * Defer the KVM_INTERNAL_ERROR exit until
> > > +                      * someone tries to trigger posted interrupt
> > > +                      * processing on this vCPU, to avoid breaking
> > > +                      * existing kvm-unit-tests.
> >
> > Run the lines out to 80 chars.  Also, can we change the comment to tie it to
> > CPU behavior in someway?  A few years down the road, "existing kvm-unit-tests"
> > may not have any relevant meaning, and it's not like kvm-unit-tests is bug free
> > either.  E.g. something like
> >
> >                         /*
> >                          * Defer the KVM_INTERNAL_ERROR exit until posted
> >                          * interrupt processing actually occurs on this vCPU.
> >                          * Until that happens, the descriptor is not accessed,
> >                          * and userspace can technically rely on that behavior.
> >                          */
> Okay...except for the fact that kvm will rather gratuitously process
> posted interrupts in situations where hardware won't. That makes it
> difficult to tie this to hardware behavior.

Hrm, true, but we can at say that KVM won't bail if there's zero chance of posted
interrupts being processed.  I hope?