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

On 29.08.19 14:27, Jan Beulich wrote:
On 29.08.2019 14:01, Juergen Gross wrote:
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.

That's the implication; note that I've taken the text from the
Linux commit.

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?

That's an alternative option. But if anything I'd see us providing
both, not the one you suggest instead of what the patch here does.

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

I'm not at all sure about this.

Suspend/resume in Xen was broken for more than 3 months in the late 4.12
development phase and nobody noticed it. Only when I started my core
scheduling series I came across the issue.


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