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

[Joerg Roedel]

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.


Regards,

	Joerg