On Wed, Jun 29, 2022 at 07:45:07PM +0100, Andrew Cooper wrote:
> Back at the time of the original Spectre-v2 fixes, it was recommended to clear
> MSR_SPEC_CTRL when going idle. This is because of the side effects on the
> sibling thread caused by the microcode IBRS and STIBP implementations which
> were retrofitted to existing CPUs.
>
> However, there are no relevant cross-thread impacts for the hardware
> IBRS/STIBP implementations, so this logic should not be used on Intel CPUs
> supporting eIBRS, or any AMD CPUs; doing so only adds unnecessary latency to
> the idle path.
>
> Furthermore, there's no point playing with MSR_SPEC_CTRL in the idle paths if
> SMT is disabled for other reasons.
>
> Fixes: 8d03080d2a33 ("x86/spec-ctrl: Cease using thunk=lfence on AMD")
> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
Reviewed-by: Roger Pau Monné <roger....@citrix.com>
Thanks, Roger.
