Re: [PATCH 03/28] KVM: nSVM: inject exceptions via svm_check_nested_events

[Paolo Bonzini]

On 08/03/21 21:43, Sean Christopherson wrote:
> On Mon, Mar 08, 2021, Paolo Bonzini wrote:
> > On 08/03/21 17:44, Sean Christopherson wrote:
> > > VMCALL is also probably ok
> > > in most scenarios, but patching L2's code from L0 KVM is sketchy.
> >
> > I agree that patching is sketchy and I'll send a patch.  However...
> >
> > > > The same is true for the VMware #GP interception case.
> > >
> > > I highly doubt that will ever work out as intended for the modified IO #GP
> > > behavior.  The only way emulating #GP in L2 is correct if L1 wants to pass
> > > through the capabilities to L2, i.e. the I/O access isn't intercepted by L1.
> > > That seems unlikely.
> >
> > ... not all hypervisors trap everything.  In particular in this case the
> > VMCS12 I/O permission bitmap should be consulted (which we do in
> > vmx_check_intercept_io), but if the I/O is not trapped by L1 it should
> > bypass the IOPL and TSS-bitmap checks in my opinion.
>
> I agree, _if_ it's not trapped.  But bypassing the checks when it is trapped is
> clearly wrong.

You can still trap #GP unconditionally and run the emulator. The 
intercept check will (or should) handle it.

Paolo