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

[Joerg Roedel]

Hi Peter,

On Tue, Jun 23, 2020 at 12:45:59PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 23, 2020 at 11:45:19AM +0200, Joerg Roedel wrote:
> > Or maybe you have a better idea how to implement this, so I'd like to
> > hear your opinion first before I spend too many days implementing
> > something.
> 
> OK, excuse my ignorance, but I'm not seeing how that IST shifting
> nonsense would've helped in the first place.
> 
> If I understand correctly the problem is:
> 
> 	<#VC>
> 	  shift IST
> 	  <NMI>
> 	    ... does stuff
> 	    <#VC> # again, safe because the shift
> 
> But what happens if you get the NMI before your IST adjustment?

The v3 patchset implements an unconditional shift of the #VC IST entry
in the NMI handler, before it can trigger a #VC exception.

> Either way around we get to fix this up in NMI (and any other IST
> exception that can happen while in #VC, hello #MC). And more complexity
> there is the very last thing we need :-(

Yes, in whatever way this gets implemented, it needs some fixup in the
NMI handler. But that can happen in C code, so it does not make the
assembly more complex, at least.

> There's no way you can fix up the IDT without getting an NMI first.

Not sure what you mean by this.

> This entire exception model is fundamentally buggered :-/

Regards,

	Joerg