Re: [PATCH v4 10/12] KVM: x86: never write to memory from kvm_vcpu_check_block()

[Sean Christopherson]

On Thu, Dec 14, 2023, Maxim Levitsky wrote:
> On Tue, 2023-12-12 at 07:28 -0800, Sean Christopherson wrote:
> > On Sun, Dec 10, 2023, Jim Mattson wrote:
> > > On Thu, Dec 7, 2023 at 8:21 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > > Doh.  We got the less obvious cases and missed the obvious one.
> > > > 
> > > > Ugh, and we also missed a related mess in kvm_guest_apic_has_interrupt().  That
> > > > thing should really be folded into vmx_has_nested_events().
> > > > 
> > > > Good gravy.  And vmx_interrupt_blocked() does the wrong thing because that
> > > > specifically checks if L1 interrupts are blocked.
> > > > 
> > > > Compile tested only, and definitely needs to be chunked into multiple patches,
> > > > but I think something like this mess?
> > > 
> > > The proposed patch does not fix the problem. In fact, it messes things
> > > up so much that I don't get any test results back.
> > 
> > Drat.
> > 
> > > Google has an internal K-U-T test that demonstrates the problem. I
> > > will post it soon.
> > 
> > Received, I'll dig in soonish, though "soonish" might unfortunately might mean
> > 2024.
> > 
> 
> Hi,
> 
> So this is what I think:
> 
> KVM does have kvm_guest_apic_has_interrupt() for this exact purpose,
> to check if nested APICv has a pending interrupt before halting.

For all intents and purposes, so was nested_ops->has_events().  I don't see
any reason to have two APIs that do the same thing, and the call to
kvm_guest_apic_has_interrupt() is wrong in that it doesn't verify that IRQs are
enabled for _L2_.  That's why my preference is to fold the two together.

> However the problem is bigger - with APICv we have in essence 2 pending
> interrupt bitmaps - the PIR and the IRR, and to know if the guest has a
> pending interrupt one has in theory to copy PIR to IRR, then see if the max
> is larger then the current PPR.

Yeah, this is what my untested hack-a-patch tried to do.

> Since we don't want to write to guest memory,

The changelog is misleading/wrong.  Writing guest memory is ok, what isn't safe
is blocking or sleeping, i.e. KVM must not trigger a host page fault due to
accessing a page that's been swapped out.  Read vs. write doesn't matter.

So KVM can safely read and write guest memory so long as it already mapped by 
kvm_vcpu_map() (or I suppose if we wrapped an access with pagefault_disable(),
but I can't think of a sane reason to do that).  E.g. nVMX can access a vCPU's
PID mapping, but synthesizing a nested VM-Exit will cause explosions on nSVM.

> and the IRR here resides in the guest memory, I guess we have to do a
> 'dry-run' version of 'vmx_complete_nested_posted_interrupt' and call it from
> kvm_guest_apic_has_interrupt().

nested_ops->has_events() is the much better fit, e.g. the naming won't get weird
and we can gate the whole thing on is_guest_mode().  Though we probably need a
wrapper to handle any commonalities between nVMX and nSVM.

> What do you think? I can prepare a patch for this.

As above, this is what I tried to do, sort of.  Though it's obviously broken.  We
don't need a full dry-run because KVM only needs to detect events that are unique
to L2, e.g. nVMX's preemption timer, MTF, and pending virtual interrupts (hmm,
I suspect nSVM's vNMI is broken too).  Things like INIT and SMI don't require
nested virtualization awareness because the event itself is tracked for the vCPU
as a whole.