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

Peter Zijlstra
2020-06-23T12:52:33Z

On Tue, Jun 23, 2020 at 02:04:33PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 01:48:18PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote:
>
> > But you cannot do a recursion check in #VC, because the NMI can happen
> > on the first instruction of #VC, before we can increment our counter,
> > and then the #VC can happen on NMI because the IST stack is a goner, and
> > we're fscked again (or on a per-cpu variable we touch in our elaborate
> > NMI setup, etc..).
>
> No, the recursion check is fine, because overwriting an already used IST
> stack doesn't matter (as long as it can be detected) if we are going to
> panic anyway. It doesn't matter because the kernel will not leave the
> currently running handler anymore.

You only have that guarantee when any SNP #VC from kernel is an
automatic panic. But in that case, what's the point of having the
recursion count?

> > I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in
> > specific is utter crap.
>
> I agree, but don't forget the most prominent underlying reason for IST:
> The SYSCALL gap. If SYSCALL would switch stacks most of those issues
> would not exist. IST would still be needed because there are no task
> gates in x86-64, but still...

We could all go back to int80 ;-) /me runs like heck