linux-kernel - Re: [PATCH] x86/AMD: Apply erratum 688 on machines without a BIOS fix

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Date:   Mon, 24 Oct 2016 11:35:10 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     sonofagun@...nmailbox.org
Cc:     linux-kernel@...r.kernel.org, Nikos Barkas <levelwol@...il.com>
Subject: Re: [PATCH] x86/AMD: Apply erratum 688 on machines without a BIOS fix

On Mon, Oct 24, 2016 at 02:38:06PM +0300, sonofagun@...nmailbox.org wrote:
> The patch is not equivalent to the original. As a result it behaves
> differently. To be specific, using dmesg I get the expected value from the
> affected MSR with the original patch. With the latest patch, patching of the
> MSR occurs after dmesg prints the MSR information. That is why I thought it
> did nothing.

Gah, that "show_msr" is crap - it gets issued too early and we can -
and we do - set MSRs later too. Oh and it prints only the BSP. I should
probably rip it out - there's msr-tools for that which is much better.

> rdmsr --all 0xc0011021 returns the expected results on all CPUs with both
> patches. I have the impression that the system boots slower because the fix
> is applied later compared to the original patch.

Could be - setting those bits 3 in 14 in that MSR is probably disabling
some hw features which may impact performance.

> Could you please use perf and tell me what values do you get at perf
> branch-misses right after boot on your ON-B0 box? Launching firefox with
> only one tab gives you similar numbers?

Sure, give me the exact command you're executing so that I can do it here.

> If you need anything more, feel free to ask.

Out of pure interest: do you remember how exactly you did reproduce this
issue?

Thanks.

-- 
Regards/Gruss,
    Boris.

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