linux-kernel - Re: Question about x86/mm/gup.c's use of disabled interrupts

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Message-ID: <49C17BD8.6050609@goop.org>
Date:	Wed, 18 Mar 2009 15:55:20 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Jan Beulich <jbeulich@...ell.com>, Ingo Molnar <mingo@...e.hu>
Subject: Re: Question about x86/mm/gup.c's use of disabled interrupts

Avi Kivity wrote:
>> Hm, awkward if flush_tlb_others doesn't IPI...
>>
>
> How can it avoid flushing the tlb on cpu [01]?  It's it's 
> gup_fast()ing a pte, it may as well load it into the tlb.

xen_flush_tlb_others uses a hypercall rather than an IPI, so none of the 
logic which depends on there being an IPI will work.

>> Simplest fix is to make gup_get_pte() a pvop, but that does seem like 
>> putting a red flag in front of an inner-loop hotspot, or something...
>>
>> The per-cpu tlb-flush exclusion flag might really be the way to go.
>
> I don't see how it will work, without changing Xen to look at the flag?
>
> local_irq_disable() is used here to lock out a remote cpu, I don't see 
> why deferring the flush helps.

Well, no, not deferring.  Making xen_flush_tlb_others() spin waiting for 
"doing_gup" to clear on the target cpu.  Or add an explicit notion of a 
"pte update barrier" rather than implicitly relying on the tlb IPI 
(which is extremely convenient when available...).

    J
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