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

Peter Zijlstra
2020-06-23T13:59:56Z

On Tue, Jun 23, 2020 at 03:40:03PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 02:52:01PM +0200, Peter Zijlstra wrote:

> > 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?
>
> It is not a recursion count, it is a stack-recursion check. Basically
> walk down the stack and look if your current stack is already in use.
> Yes, this can be optimized, but that is what is needed.
>
> IIRC the current prototype code for SNP just pre-validates all memory in
> the VM and doesn't support moving pages around on the host. So any #VC
> SNP exception would be fatal, yes.
>
> In a scenario with on-demand validation of guest pages and support for
> guest-assisted page-moving on the HV side it would be more complicated.
> Basically all memory that is accessed during #VC exception handling must
> stay validated at all times, including the IST stack.
>
> So saying this, I don't understand why _all_ SNP #VC exceptions from
> kernel space must be fatal?

Ah, because I hadn't thought of the stack-recursion check.

So basically when your exception frame points to your own IST, you die.
That sounds like something we should have in generic IST code.