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Message-ID: <20091102161248.GB15423@elte.hu>
Date: Mon, 2 Nov 2009 17:12:48 +0100
From: Ingo Molnar <mingo@...e.hu>
To: Gleb Natapov <gleb@...hat.com>
Cc: kvm@...r.kernel.org, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Frédéric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 02/11] Add "handle page fault" PV helper.
* Gleb Natapov <gleb@...hat.com> wrote:
> On Mon, Nov 02, 2009 at 10:22:14AM +0100, Ingo Molnar wrote:
> >
> > * Gleb Natapov <gleb@...hat.com> wrote:
> >
> > > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > > index f4cee90..14707dc 100644
> > > --- a/arch/x86/mm/fault.c
> > > +++ b/arch/x86/mm/fault.c
> > > @@ -952,6 +952,9 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
> > > int write;
> > > int fault;
> > >
> > > + if (arch_handle_page_fault(regs, error_code))
> > > + return;
> > > +
> >
> > This patch is not acceptable unless it's done cleaner. Currently we
> > already have 3 callbacks in do_page_fault() (kmemcheck, mmiotrace,
> > notifier), and this adds a fourth one. Please consolidate them into a
> > single callback site, this is a hotpath on x86.
> >
> This call is patched out by paravirt patching mechanism so overhead
> should be zero for non paravirt cases. [...]
arch_handle_page_fault() isnt upstream yet - precisely what is the
instruction sequence injected into do_page_fault() in the patched-out
case?
> [...] What do you want to achieve by consolidate them into single
> callback? [...]
Less bloat in a hotpath and a shared callback infrastructure.
> [...] I mean the code will still exist and will have to be executed on
> every #PF. Is the goal to move them out of line?
The goal is to have a single callback site for all the users - which
call-site is patched out ideally - on non-paravirt too if needed. Most
of these callbacks/notifier-chains have are inactive most of the time.
I.e. a very low overhead 'conditional callback' facility, and a single
one - not just lots of them sprinkled around the code.
Ingo
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