Re: [PATCH v3 22/22] kvm: x86: Disable interception for IA32_XFD on demand

[Sean Christopherson <seanjc@google.com>]

On Wed, Dec 29, 2021, Tian, Kevin wrote:
> > From: Tian, Kevin
> > Sent: Wednesday, December 29, 2021 11:35 AM
> > >
> > > Speaking of nested, interception of #NM in
> > vmx_update_exception_bitmap()
> > > is wrong
> > > with respect to nested guests.  Until XFD is supported for L2, which I didn't
> > > see
> > > in this series, #NM should not be intercepted while L2 is running.
> > 
> > Can you remind what additional thing is required to support XFD for L2?
> 
> ok, at least we require additional work on when to disable write interception.
> It can be done only when both L0 and L1 make the same decision, just like 
> what we did for many other VMCS settings...

I'm not terribly concerned with exposing XFD to L2 right now, I'm much more
concerned with not breaking nVMX when XFD is exposed to L1.

> > If only about performance I prefer to the current conservative approach
> > as the first step. As explained earlier, #NM should be rare if the guest
> > doesn't run AMX applications at all. Adding nested into this picture doesn't
> > make things a lot worser.
> 
> All your comments incorporated except this one. As said always trapping
> #NM for L2 is not a big problem, as long as it's rare and don't break function.
> Usually a relatively static scheme is safer than a dynamic one in case of
> anything overlooked for the initial implementation. 😊

I strongly disagree, it's not automagically safer.  Either way, we need to validate
that KVM does the correct thing with respect to vmcs12.  E.g. does KVM correctly
reflect the #NM back to L2 when it's not intercepted by L1?  Does it synthesize a
nested VM-Exit when it is intercepted by L1?  The testing required doesn't magically
go away.

As posted, there is zero chance that the patches correctly handling #NM in L2
because nested_vmx_l0_wants_exit() doesn't prevent an #NM from being forwarded
to L1.  E.g. if L0 (KVM) and L1 both intercept #NM, handle_exception_nm() will
queue a #NM for injection and then syntehsizea nested VM-Exit, i.e. the #NM from
L2 will get injected into L1 after the nested exit.

That also means handle_exception_nmi_irqoff() => handle_exception_nm() is fatally
broken on non-XFD hardware, as it will attempt RDMSR(MSR_IA32_XFD_ERR) if L1
intercepts #NM since handle_exception_nmi_irqoff() will run before
__vmx_handle_exit() => nested_vmx_reflect_vmexit() checks whether L0 or L1 should
handle the exit.