Re: [PATCH v5] x86: clear RDRAND CPUID bit on AMD family 15h/16h

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 18/05/2020 14:19, 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.
>
> Warn if no explicit choice was given on affected hardware.
>
> Check RDRAND functions at boot as well as after S3 resume (the retry
> limit chosen is entirely arbitrary).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

> ---
> Still slightly RFC, and still in particular because of the change to
> parse_xen_cpuid(): Alternative approach suggestions are welcome. But now
> also because with many CPUs there may now be a lot of warnings in case
> of issues.

It would still be nice if we could find a better way of determining
whether S3 is supported on the platform, which would at least let us
sort server and client platforms.

A straight string search for _S3 in the DSDT does look to be effective,
on a sample of 5 boxes I've tried.

~Andrew