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Re: [PATCH] x86/ucode/AMD: apply the patch early on every logical thread
- To: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Thu, 5 Jan 2023 22:56:06 +0000
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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- Thread-topic: [PATCH] x86/ucode/AMD: apply the patch early on every logical thread
On 05/01/2023 1:20 pm, Sergey Dyasli wrote:
> The original issue has been reported on AMD Bulldozer-based CPUs where
> ucode loading loses the LWP feature bit in order to gain the IBPB bit.
> LWP disabling is per-SMT core modification and needs to happen on each
> sibling SMT thread despite the shared microcode engine. Otherwise,
> logical CPUs will end up with different cpuid capabilities.
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=216211
Bulldozer is CMT, not SMT. The relevant property is that both CMT's
share a single microcode sequencer.
Xen happens to not be impacted by that specific bug, because we level
the feature masking/override MSRs, and the LWP bit falls out.
It's also important to state that loading on every logical processor is
the recommendation from AMD, after discussing that bug. Because it
contradicts the currently written advice in the BKDG/PPRs.
> diff --git a/xen/arch/x86/cpu/microcode/private.h
> b/xen/arch/x86/cpu/microcode/private.h
> index 73b095d5bf..c4c6729f56 100644
> --- a/xen/arch/x86/cpu/microcode/private.h
> +++ b/xen/arch/x86/cpu/microcode/private.h
> @@ -7,6 +7,7 @@ extern bool opt_ucode_allow_same;
>
> enum microcode_match_result {
> OLD_UCODE, /* signature matched, but revision id is older or equal */
> + SAME_UCODE, /* signature matched, but revision id is the same */
> NEW_UCODE, /* signature matched, but revision id is newer */
> MIS_UCODE, /* signature mismatched */
> };
I don't think this is a clever idea. For one, OLD and SAME are now
ambiguous (at least as far as the comments go), and having the
difference between the two depend on allow_same is unexpected to say the
least.
I never really liked the enum to begin with, and I think the logic would
be cleaner without it.
We depend entirely on there being one ucode blob which is applicable
globally across the system, so MIS_UCODE can be expressed as returning
NULL from the initial searches. Everything else can then be expressed
in a normal {mem,str}cmp() way (i.e. -1/0/+1).
~Andrew
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