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Re: [PATCH 6/6] x86/spec-ctrl: Introduce and use DO_COND_BHB_SEQ
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 30 Apr 2024 11:20:21 +0200
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 30 Apr 2024 09:20:32 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.04.2024 20:14, Andrew Cooper wrote:
> @@ -322,12 +334,9 @@
> ALTERNATIVE "", __stringify(DO_SPEC_CTRL_ENTRY maybexen=1), \
> X86_FEATURE_SC_MSR_PV
>
> - testb $SCF_entry_bhb, %bl
> - jz .L\@_skip_bhb
> - ALTERNATIVE_2 "", \
> - "call clear_bhb_loops", X86_SPEC_BHB_LOOPS, \
> - "call clear_bhb_tsx", X86_SPEC_BHB_TSX
> -.L\@_skip_bhb:
> + ALTERNATIVE_2 "", \
> + "DO_COND_BHB_SEQ clear_bhb_loops", X86_SPEC_BHB_LOOPS, \
> + "DO_COND_BHB_SEQ clear_bhb_tsx", X86_SPEC_BHB_TSX
Only spotting this while doing the 456 backport to 4.9: While this
is all usual and fine, ...
> @@ -433,13 +442,9 @@
> * Clear the BHB to mitigate BHI. Used on eIBRS parts, and uses RETs
> * itself so must be after we've perfomed all the RET-safety we can.
> */
> - testb $SCF_entry_bhb, %bl
> - jz .L\@_skip_bhb
> -
> - ALTERNATIVE_2 "", \
> - "call clear_bhb_loops", X86_SPEC_BHB_LOOPS, \
> - "call clear_bhb_tsx", X86_SPEC_BHB_TSX
> -.L\@_skip_bhb:
> + ALTERNATIVE_2 "", \
> + "DO_COND_BHB_SEQ clear_bhb_loops", X86_SPEC_BHB_LOOPS, \
> + "DO_COND_BHB_SEQ clear_bhb_tsx", X86_SPEC_BHB_TSX
>
> lfence
... the original use of ALTERNATIVE_2 here is safe only because for
the idle domain SCF_entry_bhb will not be set, and hence any patching
done here will not affect NMI or #MC taken while doing the patching.
Therefore I think this hunk needs dropping.
Jan
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