linux-kernel - Re: [RFC][PATCH 2/6] mm: Change flush_tlb_range() to take an mm

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Date:	Thu, 03 Mar 2011 10:45:42 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	cmetcalf@...era.com
Cc:	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	aarcange@...hat.com, tglx@...utronix.de, riel@...hat.com,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org, benh@...nel.crashing.org,
	hugh.dickins@...cali.co.uk, mel@....ul.ie, npiggin@...nel.dk,
	rmk@....linux.org.uk, schwidefsky@...ibm.com
Subject: Re: [RFC][PATCH 2/6] mm: Change flush_tlb_range() to take an
 mm_struct

From: Chris Metcalf <cmetcalf@...era.com>
Date: Thu, 3 Mar 2011 12:22:37 -0500

> I'm finding it hard to understand how the Sparc code handles icache
> coherence.  It seems that the Spitfire MMU is the interesting one, but the
> hard case seems to be when a process migrates around to various cores
> during execution (thus leaving incoherent icache lines everywhere), and the
> page is then freed and re-used for different executable code.  I'd think
> that there would have to be xcall IPIs to flush all the cpus' icaches, or
> to flush every core in the cpu_vm_mask plus do something at context switch,
> but I don't see any of that.  No doubt I'm missing something :-)

flush_dcache_page() remembers the cpu that wrote to the page (in the
page flags), and cross-calls to that specific cpu.

It is only that cpu which must flush his I-cache, since all other cpus
saw the write on the bus and updated their I-cache lines as a result.

See, in the sparc64 case, the incoherency issue is purely local to the
store.  The problem case is specifically the local I-cache not seeing
local writes, everything else is fine.  CPU I-caches see writes done
by other cpus, just not those done by the local cpu.

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