Linux Virtualization - Jun 2020 - Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)

On Tue, Jun 23, 2020 at 01:14:43PM +0200, Peter Zijlstra
wrote:
> On Tue, Jun 23, 2020 at 01:11:07PM +0200, Joerg Roedel wrote:
> > The v3 patchset implements an unconditional shift of the #VC IST entry
> > in the NMI handler, before it can trigger a #VC exception.
> 
> Going by that other thread -- where you said that any memory access can
> trigger a #VC, there just isn't such a guarantee.
As I wrote in the other mail, this can only happen when SNP gets enabled
(which is follow-on work to this) and is handled by a stack recursion
check in the #VC handler.

The reason I mentioned the #VC-anywhere case is to make it more clear
why #VC needs an IST handler.

Regards,

	Joerg

On Tue, Jun 23, 2020 at 01:43:24PM +0200, Joerg Roedel
wrote:
> On Tue, Jun 23, 2020 at 01:14:43PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 01:11:07PM +0200, Joerg Roedel wrote:
> 
> > > The v3 patchset implements an unconditional shift of the #VC IST
entry
> > > in the NMI handler, before it can trigger a #VC exception.
> > 
> > Going by that other thread -- where you said that any memory access
can
> > trigger a #VC, there just isn't such a guarantee.
> 
> As I wrote in the other mail, this can only happen when SNP gets enabled
> (which is follow-on work to this) and is handled by a stack recursion
> check in the #VC handler.
> 
> The reason I mentioned the #VC-anywhere case is to make it more clear
> why #VC needs an IST handler.
If SNP is the sole reason #VC needs to be IST, then I'd strongly urge
you to only make it IST if/when you try and make SNP happen, not before.

On Tue, Jun 23, 2020 at 01:50:14PM +0200, Peter Zijlstra
wrote:
> If SNP is the sole reason #VC needs to be IST, then I'd strongly urge
> you to only make it IST if/when you try and make SNP happen, not before.
It is not the only reason, when ES guests gain debug register support
then #VC also needs to be IST, because #DB can be promoted into #VC
then, and as #DB is IST for a reason, #VC needs to be too.

Besides that, I am not a fan of delegating problems I already see coming
to future-Joerg and future-Peter, but if at all possible deal with them
now and be safe later.

Regards,

	Joerg