linux-kernel - Re: [patch] x86, mm: avoid stale tlb entries by clearing prev mm

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Date:	Thu, 3 Feb 2011 11:48:41 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Suresh Siddha <suresh.b.siddha@...el.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Mallick, Asit K" <asit.k.mallick@...el.com>
Subject: Re: [patch] x86, mm: avoid stale tlb entries by clearing prev
 mm_cpumask after switching mm

On Thu, Feb 3, 2011 at 11:34 AM, Suresh Siddha
<suresh.b.siddha@...el.com> wrote:
>
> True. 'stale' is the wrong word. Do you want me to send a corrected one
> by replacing it with 'bogus'?

Please.

> my understanding is that unless we end up using that TLB entry, we will
> not have the issues like machine checks due to cacheability issues etc.
> If it is not global, upcoming cr3 change will flush it and meanwhile I
> don't think there is a scenario where we refer to these user-addresses.

Quite possible. The situation I envisioned was the same speculative
memory access that causes the TLB fill to also cause a cache fill -
for a noncacheable region (because the bogus TLB entry sets the random
address to cacheable).

And then what happens when somebody else accesses the same memory
noncacheably (through a valid TLB entry), and finds it in the cache?

I dunno. Not really important. The important part is the "possible
random bogus TLB entry", the fact that the CPU can act strangely after
that is pretty much a given.

                            Linus
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