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SubjectRe: Should SEV-ES #VC use IST? (Re: [PATCH] Allow RDTSC and RDTSCP from userspace)
On Tue, Jun 23, 2020 at 01:30:07PM +0200, Joerg Roedel wrote:
> Note that this is an issue only with secure nested paging (SNP), which
> is not enabled yet with this patch-set. When it gets enabled a stack
> recursion check in the #VC handler is needed which panics the VM. That
> also fixes the #VC-in-early-NMI problem.

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..).

There is no way I can see SNP-#VC 'work'. The best I can come up with is
'mostly', but do you like your bridges/dikes/etc.. to be mostly ok? Or
do you want a guarantee they'll actually work?

I'll keep repeating this, x86_64 exceptions are a trainwreck, and IST in
specific is utter crap.

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