linux-kernel - Re: [PATCH 0/15] KVM: optimize for MMIO handled

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Message-ID: <4DF0789B.2050401@redhat.com>
Date:	Thu, 09 Jun 2011 10:39:07 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH 0/15] KVM: optimize for MMIO handled

On 06/07/2011 03:58 PM, Xiao Guangrong wrote:
> The idea of this patchset is from Avi:
> | We could cache the result of a miss in an spte by using a reserved bit, and
> | checking the page fault error code (or seeing if we get an ept violation or
> | ept misconfiguration), so if we get repeated mmio on a page, we don't need to
> | search the slot list/tree.
> | (https://lkml.org/lkml/2011/2/22/221)
>
> The aim of this patchset is to support fast mmio emulate, it reduce searching
> mmio gfn from memslots which is very expensive since we need to walk all slots
> for mmio gfn, and the other advantage is: we can reduce guest page table walking
> for soft mmu.
>
> Lockless walk shadow page table is introduced in this patchset, it is the light
> way to check the page fault is the real mmio page fault or something is running
> out of our mind.
>
> And, if shadow_notrap_nonpresent_pte is enabled(bypass_guest_pf=1), mmio page
> fault and normal page fault is mixed(the reserved is set for all page fault),
> it has little regression, if the box can generate lots of mmio access, for
> example, the network server, it can disable shadow_notrap_nonpresent_pte and
> enable mmio pf, after all, we can enable/disable mmio pf at the runtime.
>

Okay, this is pretty complicated.  And things are already complicated.

First, I think we should consider dropping bypass_guest_pf completely, 
just so we have less things to think about.

Second, I don't like two paths for accessing shadow page tables, it 
makes the code much larger.  I'm also not sure RCU is enough protection 
- we can unlink a page in the middle of a hierarchy, and on i386 this 
causes an invalid pointer to appear when we fetch the two halves.  But I 
guess, if the cpu can do it, so can we.

Maybe we can do something like

again:
   fetch pointer to last level spte using RCU
   if failed:
        take lock
        build spte hierarchy
        drop lock
        goto again
   if sync:
       if mmio:
           do mmio
           return
       return
   walk guest table
   install spte
   if mmio:
      do mmio

(sync is always false for tdp)

-- 
error compiling committee.c: too many arguments to function

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