linux-kernel - Re: [PATCH 2/2] x86/barrier: Do not serialize MSR accesses on AMD

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Date:   Thu, 2 Nov 2023 12:08:48 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Andrew Cooper <andrew.cooper3@...rix.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, X86 ML <x86@...nel.org>,
        Kishon VijayAbraham <Kishon.VijayAbraham@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] x86/barrier: Do not serialize MSR accesses on AMD

On Fri, Oct 27, 2023 at 09:09:05PM +0100, Andrew Cooper wrote:
> Quoteth the APM (rev 3.41, June 2023):
> 
> 16.11.2 WRMSR / RDMSR serialization for x2APIC Register
> 
> The WRMSR instruction is used to write the APIC register set in x2APIC
> mode. Normally WRMSR is
> a serializing instruction, however when accessing x2APIC registers, the
> serializing aspect of WRMSR
> is relaxed to allow for more efficient access to those registers.
> Consequently, a WRMSR write to an
> x2APIC register may complete before older store operations are complete
> and have become globally
> visible. When strong ordering of an x2APIC write access is required with
> respect to preceding memory
> operations, software can insert a serializing instruction (such as
> MFENCE) before the WRMSR
> instruction.
> 
> So which is right?  This commit message, or the APM?  (and yes, if
> you're waiting on an APM update then the commit message should at least
> note that one is coming.)

To clarify this one: there will be a CPUID bit which states that an
MFENCE is not needed ATM and it'll be added to the APM. I'll add a note
about it to the commit message too.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette