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:

quoted

On Tue, Apr 28, 2020 at 05:16:57PM +0200, Joerg Roedel wrote:

quoted

+	/*
+	 * 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.