Re: [Xen-devel] [PATCH] x86: clear RDRAND CPUID bit on AMD family 15h/16h

[Juergen Gross <jgross@xxxxxxxx>]

On 29.08.19 13:42, Andrew Cooper wrote:
> On 29/08/2019 10:28, Jan Beulich wrote:
> > Inspired by Linux commit c49a0a80137c7ca7d6ced4c812c9e07a949f6f24:
> >
> >      There have been reports of RDRAND issues after resuming from suspend on
> >      some AMD family 15h and family 16h systems. This issue stems from a BIOS
> >      not performing the proper steps during resume to ensure RDRAND continues
> >      to function properly.
> >
> >      Update the CPU initialization to clear the RDRAND CPUID bit for any family
> >      15h and 16h processor that supports RDRAND. If it is known that the family
> >      15h or family 16h system does not have an RDRAND resume issue or that the
> >      system will not be placed in suspend, the "cpuid=rdrand" kernel parameter
> >      can be used to stop the clearing of the RDRAND CPUID bit.
> >
> >      Note, that clearing the RDRAND CPUID bit does not prevent a processor
> >      that normally supports the RDRAND instruction from executing it. So any
> >      code that determined the support based on family and model won't #UD.
Yeah, but it will no longer see random numbers after resume.

> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > ---
> > Slightly RFC, in particular because of the change to parse_xen_cpuid():
> > Alternative approach suggestions are welcome.
> This issue was on my radar, but I hadn't got around to starting a patch yet.
>
> I was wondering if we could perhaps default it to whether S3 was
> available, but looking at the code which prints "ACPI sleep modes: S3",
> it doesn't appear to be related to any real ACPI information.
Wouldn't it make more sense to inhibit suspend/resume instead?

I'm quite sure a machine running Xen is very rarely put to S3.


Juergen

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