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Re: [XEN PATCH 2/2] x86: Refactor LBR feature to MSR_DEBUGCTL feature
- To: Matthew Barnes <matthew.barnes@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 25 Mar 2024 17:29:18 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 25 Mar 2024 16:29:27 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15.03.2024 18:52, Matthew Barnes wrote:
> Last Branch Record and Bus Lock Detect both belong to the same MSR.
>
> The same mechanism that restores LBR also restores BLD.
>
> Therefore, the name of the feature that enables this mechanism should
> reflect restoring the MSR, instead of one field.
>
> No functional change.
>
> Signed-off-by: Matthew Barnes <matthew.barnes@xxxxxxxxx>
So this in part is what I asked about in reply to patch 1. However,
afaict the order of patches wants flipping and ...
> @@ -2129,10 +2129,10 @@ void percpu_traps_init(void)
> return;
> }
>
> - setup_force_cpu_cap(X86_FEATURE_XEN_LBR);
> + setup_force_cpu_cap(X86_FEATURE_XEN_MSR_DEBUGCTL);
... besides here you also want to engage the feature when using BLD
(in the other patch, once that comes comes on top of this one).
> }
>
> - if ( cpu_has_xen_lbr )
> + if ( cpu_has_xen_msr_debugctl )
> {
> host_msr_debugctl |= IA32_DEBUGCTLMSR_LBR;
> wrmsrl(MSR_IA32_DEBUGCTLMSR, host_msr_debugctl);
This can't be quite right - LBR shouldn't be enabled without the
respective command line option present.
And btw - I don't think there's a need for the _msr_ infix in the new
names you use.
Jan
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