linux-kernel - Re: [patch v3 4/7] x86/smp: Use dedicated cache-line for mwait_play

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Message-ID: <20230620090157.GVZJFrBYuN/Wr/9N9L@fat_crate.local>
Date:   Tue, 20 Jun 2023 11:01:57 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Mario Limonciello <mario.limonciello@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Tony Battersby <tonyb@...ernetics.com>,
        Ashok Raj <ashok.raj@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Arjan van de Veen <arjan@...ux.intel.com>,
        Eric Biederman <ebiederm@...ssion.com>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [patch v3 4/7] x86/smp: Use dedicated cache-line for
 mwait_play_dead()

On Thu, Jun 15, 2023 at 10:33:55PM +0200, Thomas Gleixner wrote:
> Monitoring idletask::thread_info::flags in mwait_play_dead() has been an
> obvious choice as all what is needed is a cache line which is not written
> by other CPUs.
> 
> But there is a use case where a "dead" CPU needs to be brought out of that
> mwait(): kexec().

s/mwait()/wait state/

I guess.

> The CPU needs to be brought out of mwait before kexec() as kexec() can

Ditto.

> overwrite text, pagetables, stacks and the monitored cacheline of the
> original kernel.

Yikes.

> The latter causes mwait to resume execution which
> obviously causes havoc on the kexec kernel which results usually in triple
> faults.

This sounds like a stable fix, no?

Reviewed-by: Borislav Petkov (AMD) <bp@...en8.de>

-- 
Regards/Gruss,
    Boris.

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