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

Joerg Roedel
2020-04-28T07:55:20Z

On Mon, Apr 27, 2020 at 10:37:41AM -0700, Andy Lutomirski wrote:
> I have a somewhat serious question: should we use IST for #VC at all?
> As I understand it, Rome and Naples make it mandatory for hypervisors
> to intercept #DB, which means that, due to the MOV SS mess, it's sort
> of mandatory to use IST for #VC. But Milan fixes the #DB issue, so,
> if we're running under a sufficiently sensible hypervisor, we don't
> need IST for #VC.

The reason for #VC being IST is not only #DB, but also SEV-SNP. SNP adds
page ownership tracking between guest and host, so that the hypervisor
can't remap guest pages without the guest noticing.

If there is a violation of ownership, which can happen at any memory
access, there will be a #VC exception to notify the guest. And as this
can happen anywhere, for example on a carefully crafted stack page set
by userspace before doing SYSCALL, the only robust choice for #VC is to
use IST.

Regards,

Joerg