linux-kernel - Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier in context switch

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Message-ID: <20180131004155.r2iz6vl7mmlav2yn@pd.tnic>
Date:   Wed, 31 Jan 2018 01:41:55 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     David Woodhouse <dwmw@...zon.co.uk>, arjan@...ux.intel.com,
        tglx@...utronix.de, karahmed@...zon.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org,
        pbonzini@...hat.com, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, gregkh@...ux-foundation.org,
        mingo@...nel.org, luto@...nel.org, linux@...inikbrodowski.net
Subject: Re: [PATCH] x86/speculation: Use Indirect Branch Prediction Barrier
 in context switch

On Tue, Jan 30, 2018 at 04:25:26PM -0800, Tim Chen wrote:
> As dumpable is a property in mm->flags, it affects all threads running
> on other cpus sharing the same mm.

It is not about sharing the same mm - it is about sharing the RSB. How
many logical CPUs share an RSB? If it is per core (which can have two
threads) then issuing the barrier should have effect on both threads.
Thus one barrier is enough.

IOW, the granularity is determined by how many logical CPUs share the
RSB not by how many logical CPUs share an mm.

-- 
Regards/Gruss,
    Boris.

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