linux-kernel - Re: [git pull] x86: fix global_flush

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Message-ID: <20071019120559.GA8708@one.firstfloor.org>
Date:	Fri, 19 Oct 2007 14:05:59 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>, Greg KH <gregkh@...e.de>,
	Chris Wright <chrisw@...s-sol.org>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jan Beulich <jbeulich@...ell.com>, linux-kernel@...r.kernel.org
Subject: Re: [git pull] x86: fix global_flush_tlb() bug

Thanks for catching. 

> why this bug never become prominent is a mystery - it can probably be
> explained with the (still) relative obscurity of the x86_64 architecture.

global_flush_tlb() is not very common in the big scheme of things. In a normal
system it only happens single threaded during X server startup and when
the system starts.

So while it's nasty it's unlikely to really hit people in practice.

BTW while looking I noticed this code in the vermilion driver is also
surely not correct:

        /*
         * Change caching policy of the linear kernel map to avoid
         * mapping type conflicts with user-space mappings.
         * The first global_flush_tlb() is really only there to do a global
         * wbinvd().
         */

        global_flush_tlb();

That is not what gft is guaranteed to do.

It would be probably best to just do away with g_f_t() and fold it directly into
c_p_a(). I've seen little evidence the delayed flush optimization ever made 
much difference and it seems to be misused and a source of bugs. And near all
legitimate users seem to always call it directly after c_p_a() anyways.

Besides it is grossly misnamed -- it does much more than flushing TLBs.

-Andi
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