linux-kernel - Re: Fw: [PATCH] ia64: race flushing icache in do_no

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Message-ID: <463728EB.8030308@yahoo.com.au>
Date:	Tue, 01 May 2007 21:47:55 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Rohit Seth <rohitseth@...gle.com>
CC:	'Hugh Dickins' <hugh@...itas.com>,
	'Mike Stroyan' <mike.stroyan@...com>,
	'Andrew Morton' <akpm@...ux-foundation.org>,
	"'Luck, Tony'" <tony.luck@...el.com>, linux-ia64@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Fw: [PATCH] ia64: race flushing icache in do_no_page path

Rohit Seth wrote:
> Hi Nick, 
> 
> -----Original Message-----
> From: Nick Piggin [mailto:nickpiggin@...oo.com.au] 
> Sent: Friday, April 27, 2007 11:03 PM
> To: Hugh Dickins
> Cc: rohitseth@...gle.com; Mike Stroyan; Andrew Morton; Luck, Tony;
> linux-ia64@...r.kernel.org; linux-kernel@...r.kernel.org
> Subject: Re: Fw: [PATCH] ia64: race flushing icache in do_no_page path
> 
> Hugh Dickins wrote:
> 
>>On Sat, 28 Apr 2007, Nick Piggin wrote:
>>
>>
>>>OIC, you need a virtual address to evict the icache, so you can't 
>>>flush at flush_dcache time? Or does ia64 have an instruction to flush 
>>>the whole icache? (it would be worth testing, to see how much 
>>>performance suffers).
>>
>>
>>I'm puzzled by that remark: the ia64 flush_icache_range always has a 
>>virtual address, it uses the kernel virtual address; it takes no 
>>interest in whether there's a user virtual address.
> 
> 
>>I _think_ what it is doing is actually flushing dcache lines dirtied 
>>via the kernel virtual address (yes, I think flush_icache
>>in lazy_mmu_prot_update is actually just flushing the dcache, but 
>>I could be wrong? [*]).
> 
> 
> It is invalidating any entries (containing same physical address) in both I
> and D caches.  Any dirty lines in D cache are written back to memory before
> getting invalidated (ofcourse).

OK. (should it be issuing both fc and fc.i to be robust in case a
new implementation doesn't flush the dcache with fc.i?)


>>There are supposedly no icache lines at that point[**]:
> 
> 
> For this bug to trigger there has to be a (stale) entry in icache containing
> the old contents of a page that just got updated by kernel as explicit
> copying of data (DMAs are coherent on ia64, meaning if a device were to
> write to memory then architecture guarnatees that both I and D caches are
> invalidated).

So if we have a dirty dcache line for a given physical address,
it will _always_ be the case that a subsequent icache load will
find that dirty data?

... thanks for bearing with me ;)

-- 
SUSE Labs, Novell Inc.
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