Re: [PATCH v3 47/75] x86/sev-es: Add Runtime #VC Exception Handler

Sean Christopherson
2020-06-11T17:38:55Z

On Thu, Jun 11, 2020 at 01:48:31PM +0200, Joerg Roedel wrote:
> On Sat, May 23, 2020 at 09:59:24AM +0200, Borislav Petkov wrote:
> > On Tue, Apr 28, 2020 at 05:16:57PM +0200, Joerg Roedel wrote:
> > > + /*
> > > + * Mark the per-cpu GHCBs as in-use to detect nested #VC exceptions.
> > > + * There is no need for it to be atomic, because nothing is written to
> > > + * the GHCB between the read and the write of ghcb_active. So it is safe
> > > + * to use it when a nested #VC exception happens before the write.
> > > + */
> >
> > Looks liks that is that text... support for nested #VC exceptions.
> > I'm sure this has come up already but why do we even want to support
> > nested #VCs? IOW, can we do without them first or are they absolutely
> > necessary?
> >
> > I'm guessing VC exceptions inside the VC handler but what are the
> > sensible use cases?
>
> The most important use-case is #VC->NMI->#VC. When an NMI hits while the
> #VC handler uses the GHCB and the NMI handler causes another #VC, then
> the contents of the GHCB needs to be backed up, so that it doesn't
> destroy the GHCB contents of the first #VC handling path.

Isn't it possible for the #VC handler to hit a #PF, e.g. on copy_from_user()
in insn_fetch_from_user()? If that happens, what prevents the #PF handler
from hitting a #VC? AIUI, do_vmm_communication() panics if the backup GHCB
is already in use, e.g. #VC->#PF->#VC->NMI->#VC would be fatal.