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

From: Borislav Petkov
Date: Sat Aug 17 2019 - 04:43:41 EST

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. And the words "resume the hypervisor"
already means an improbable use case. Yeah, yeah, one can close the
laptop lid of her/his F15h or F16h machine while guests are running and
when the HV resumes, those guests won't get randomness but I can't seem
to find it in myself to say, uuh, that's an important use case...


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