Re: [PATCH v4 2/6] x86/sev-es: Disable IRQs while GHCB is active

From: Joerg Roedel
Date: Fri Jun 11 2021 - 10:20:43 EST

On Fri, Jun 11, 2021 at 04:05:15PM +0200, Borislav Petkov wrote:
> On Thu, Jun 10, 2021 at 11:11:37AM +0200, Joerg Roedel wrote:
> Why not simply "sandwich" them:
> local_irq_save()
> sev_es_get_ghcb()
> ...blablabla
> sev_es_put_ghcb()
> local_irq_restore();
> in every call site?

I am not a fan of this, because its easily forgotten to add
local_irq_save()/local_irq_restore() calls around those. Yes, we can add
irqs_disabled() assertions to the functions, but we can as well just
disable/enable IRQs in them. Only the previous value of EFLAGS.IF needs
to be carried from one function to the other.

> Hmm, so why aren't you accessing/setting data->ghcb_active and
> data->backup_ghcb_active safely using cmpxchg() if this path can be
> interrupted by an NMI?

Using cmpxchg is not necessary here. It is all per-cpu data, so local to
the current cpu. If an NMI happens anywhere in sev_es_get_ghcb() it can
still use the GHCB, because the interrupted #VC handler will not start
writing to it before sev_es_get_ghcb() returned.

Problems only come up when one path starts writing to the GHCB, but that
happens long after it is marked active.