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

[Jan Beulich <jbeulich@xxxxxxxx>]

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.

Jan

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