linux-kernel - Re: arch_flush_lazy_mmu_mode() in arch/x86/mm/highmem

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Message-Id: <1226951684.6905.13.camel@bodhitayantram.eng.vmware.com>
Date:	Mon, 17 Nov 2008 11:54:44 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Jan Beulich <jbeulich@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: arch_flush_lazy_mmu_mode() in arch/x86/mm/highmem_32.c

On Mon, 2008-11-17 at 10:40 -0800, Jeremy Fitzhardinge wrote:

> Yes.  The Xen code only disables interrupts temporarily while actually 
> constructing a new multicall list member, to stop a half-constructed 
> multicall from being issued by a nested flush.  But that's very brief, 
> and cheap under Xen.

We have truly magical ways of doing the same thing.

> You could do the flush in the fault handler itself, rather than 
> vmalloc_sync_one.  If you enter the handler with outstanding updates, 
> then flush them and return.  Hm, but that only works if you're always 
> going from NP->P; if you're doing P->P updates then you may just end up 
> with stale mappings.

vmalloc_sync_one really is just the fault handler, factored out to look
nice... in any case, the faults here will aways be NP->P; once created,
the page tables handling the vmalloc area will never be released, so the
PDE never transitions from P->P or P->NP (the PTEs do).

> The Novell kernel tree.  Jan's been doggedly forward-porting the old Xen 
> patches.

Okay, that explains it... the patch sequence here contains a bit of
"fun" IIRC.

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