Re: [PATCH] x86/CPU/AMD: Clear RDRAND CPUID bit on AMD family 15h/16h

From: Thomas Gleixner
Date: Sun Aug 18 2019 - 12:32:44 EST

On Sat, 17 Aug 2019, Andrew Cooper wrote:
> On 17/08/2019 09:44, Borislav Petkov wrote:
> > On Thu, Aug 15, 2019 at 10:25:24PM +0100, Andrew Cooper wrote:
> >> I'm afraid that a number of hypervisors do write-discard, given the
> >> propensity of OSes (certainly traditionally) to go poking at bits like
> >> this without wrmsr_safe().
> >>
> >> You either need to read the MSR back and observe that the bit has really
> >> changed, or in this case as Thomas suggests, look at CPUID again (which
> >> will likely be the faster option for the non-virtualised case).
> > One thing I didn't think of when we talked about this: this happens only
> > after you resume the hypervisor.
> :) It hadn't escaped my notice, hence the intervention on this thread.
> > And the words "resume the hypervisor" already means an improbable use case.
> Qubes and OpenXT are two laptop+hypervisor oriented distros where
> suspend/resume is a big deal, and these will have to follow AMD's
> recommendation here.
> However, for servers which don't do S3/S4, we can reason about safely
> leaving RDRAND enabled, irrespective of guest configuration.

Let the administrator reason about it. Default is off for sanity sake.