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

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Date:   Tue, 4 Jul 2023 09:46:31 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     X86 ML <x86@...nel.org>,
        Kishon VijayAbraham <Kishon.VijayAbraham@....com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/barrier: Do not serialize MSR accesses on AMD

On Mon, Jul 03, 2023 at 02:54:19PM +0200, Peter Zijlstra wrote:
> So you're saying that AMD tsc_deadline and x2apic MSRs *do* imply
> ordering constraints unlike the Intel ones?

Yah, that's the default situation. Only those two - TSC_DEADLINE and
x2APIC MSRs - and on *Intel* are special.

> Can we pls haz a document link for that, also a comment?

Why document the default? The SDM is already documenting this exception.
For everything else WRMSR is serializing.

> Moving this code while changing it meant I had to look at it _3_ times
> before I spotted you changed it :/

I figured it is a simple enough patch - no need to do a sole movement
one.

> Both instructions are 3 bytes, a 6 byte nop would be better, no?

Why? You wanna save the branch insn when sending IPIs through the
x2APIC? Does that really matter? I doubt it...

> 	asm volatile (ALTERNATIVE("mfence; lfence;", "", X86_FEATURE_AMD));

There's no X86_FEATURE_AMD :)

-- 
Regards/Gruss,
    Boris.

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