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Re: [PATCH v2 5/9] x86/spec-ctrl: Record the last write to MSR_SPEC_CTRL
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 31 Jan 2022 11:20:12 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 31 Jan 2022 10:20:17 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 28.01.2022 14:29, Andrew Cooper wrote:
> In some cases, writes to MSR_SPEC_CTRL do not have interesting side effects,
> and we should implement lazy context switching like we do with other MSRs.
>
> In the short term, this will be used by the SVM infrastructure, but I expect
> to extend it to other contexts in due course.
>
> Introduce cpu_info.last_spec_ctrl for the purpose, and cache writes made from
> the boot/resume paths. The value can't live in regular per-cpu data when it
> is eventually used for PV guests when XPTI might be active.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Technically
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
But ...
> --- a/xen/arch/x86/acpi/power.c
> +++ b/xen/arch/x86/acpi/power.c
> @@ -296,7 +296,10 @@ static int enter_state(u32 state)
> ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
>
> if ( boot_cpu_has(X86_FEATURE_IBRSB) )
> + {
> wrmsrl(MSR_SPEC_CTRL, default_xen_mcu_opt_ctrl);
> + ci->last_spec_ctrl = default_xen_mcu_opt_ctrl;
> + }
... wouldn't we better have a write_spec_ctrl() helper doing both,
perhaps with the check-if-same logic added as well (overridable by
a "force" boolean input, unless the cases where the write cannot be
skipped can be entirely determined from previous and new values)?
Jan
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