Re: [PATCH] x86/cpu: Fix migration safety with X86_BUG_NULL_SEL

[Andrew Cooper]

On 01/10/2021 14:33, Jane Malalane wrote:
> Currently, Linux probes for X86_BUG_NULL_SEL unconditionally which
> makes it unsafe to migrate in a virtualised environment as the
> properties across the migration pool might differ.
>
> Zen3 adds the NullSelectorClearsBase bit to indicate that loading
> a NULL segment selector zeroes the base and limit fields, as well as
> just attributes. Zen2 also has this behaviour but doesn't have the
> NSCB bit.
>
> When virtualised, NSCB might be cleared for migration safety,
> therefore we must not probe. Always honour the NSCB bit in this case,
> as the hypervisor is expected to synthesize it in the Zen2 case.
>
> Signed-off-by: Jane Malalane <jane.malalane@xxxxxxxxxx>

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