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Re: [PATCH v2 1/2] x86/spec-ctrl: Only adjust MSR_SPEC_CTRL for idle with legacy IBRS
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 30 Jun 2022 18:39:52 +0200
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 30 Jun 2022 16:40:02 +0000
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.cooper3@xxxxxxxxxx>
Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Thanks, Roger.
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