Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the nested state

From: Maxim Levitsky
Date: Mon Sep 21 2020 - 03:53:31 EST

On Sun, 2020-09-20 at 18:42 +0200, Paolo Bonzini wrote:
> On 20/09/20 18:16, Sean Christopherson wrote:
> > > Maxim, your previous version was adding some error handling to
> > > kvm_x86_ops.set_efer. I don't remember what was the issue; did you have
> > > any problems propagating all the errors up to KVM_SET_SREGS (easy),
> > > kvm_set_msr (harder) etc.?
> > I objected to letting .set_efer() return a fault.
> So did I, and that's why we get KVM_REQ_OUT_OF_MEMORY. But it was more
> of an "it's ugly and it ought not to fail" thing than something I could
> pinpoint.
> It looks like we agree, but still we have to choose the lesser evil?
> Paolo
> > A relatively minor issue is
> > the code in vmx_set_efer() that handles lack of EFER because technically KVM
> > can emulate EFER.SCE+SYSCALL without supporting EFER in hardware. Returning
> > success/'0' would avoid that particular issue. My primary concern is that I'd
> > prefer not to add another case where KVM can potentially ignore a fault
> > indicated by a helper, a la vmx_set_cr4().

The thing is that kvm_emulate_wrmsr injects #GP when kvm_set_msr returns any non zero value,
and returns 1 which means keep on going if I understand correctly (0 is userspace exit,
negative value would be a return to userspace with an error)

So the question is if we have other wrmsr handlers which return negative value, and would
be affected by changing kvm_emulate_wrmsr to pass through the error value.
I am checking the code now.

I do agree now that this is the *correct* solution to this problem.

Best regards,
Maxim Levitsky