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

Peter Zijlstra
2020-06-23T16:02:43Z

On Tue, Jun 23, 2020 at 05:38:55PM +0200, Joerg Roedel wrote:
> On Tue, Jun 23, 2020 at 05:23:26PM +0200, Peter Zijlstra wrote:

> > Reliability of that depends on the unwinder, I wouldn't want the guess
> > uwinder to OOPS me by accident.
>
> It doesn't use the full unwinder, it just assumes that there is a
> pt_regs struct at the top of every kernel stack and walks through them
> until SP points to a user-space stack.
>
> As long as the assumption that there is a pt_regs struct on top of every
> stack holds, this should be safe. The assumption might be wrong when an
> exception happens during SYSCALL/SYSENTER entry, when the return frame
> is not written by hardware.

The IRQ and SoftIRQ stacks don't have that I think. Only the task and
exception stacks.