linux-kernel - Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

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Date:	Mon, 22 Apr 2013 10:50:36 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...hat.com>, avi.kivity@...il.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 00/15] KVM: MMU: fast zap all shadow pages

On 04/21/2013 11:24 PM, Marcelo Tosatti wrote:
> On Sun, Apr 21, 2013 at 10:09:29PM +0800, Xiao Guangrong wrote:
>> On 04/21/2013 09:03 PM, Gleb Natapov wrote:
>>> On Tue, Apr 16, 2013 at 02:32:38PM +0800, Xiao Guangrong wrote:
>>>> This patchset is based on my previous two patchset:
>>>> [PATCH 0/2] KVM: x86: avoid potential soft lockup and unneeded mmu reload
>>>> (https://lkml.org/lkml/2013/4/1/2)
>>>>
>>>> [PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes
>>>> (https://lkml.org/lkml/2013/4/1/134)
>>>>
>>>> Changlog:
>>>> V3:
>>>>   completely redesign the algorithm, please see below.
>>>>
>>> This looks pretty complicated. Is it still needed in order to avoid soft
>>> lockups after "avoid potential soft lockup and unneeded mmu reload" patch?
>>
>> Yes.
>>
>> I discussed this point with Marcelo:
>>
>> ======
>> BTW, to my honest, i do not think spin_needbreak is a good way - it does
>> not fix the hot-lock contention and it just occupies more cpu time to avoid
>> possible soft lock-ups.
>>
>> Especially, zap-all-shadow-pages can let other vcpus fault and vcpus contest
>> mmu-lock, then zap-all-shadow-pages release mmu-lock and wait, other vcpus
>> create page tables again. zap-all-shadow-page need long time to be finished,
>> the worst case is, it can not completed forever on intensive vcpu and memory
>> usage.
>>
>> I still think the right way to fix this kind of thing is optimization for
>> mmu-lock.
>> ======
>>
>> Which parts scare you? Let's find a way to optimize for it. ;). For example,
>> if you do not like unmap_memslot_rmap_nolock(), we can simplify it - We can
>> use walk_shadow_page_lockless_begin() and walk_shadow_page_lockless_end() to
>> protect spte instead of kvm->being_unmaped_rmap.
>>
>> Thanks!
> 
> Xiao,
> 
> You can just remove all shadow rmaps now that you've agreed per-memslot
> flushes are not necessary. Which then gets rid of necessity for lockless 
> rmap accesses. Right?

Hi Marcelo,

I am worried about:

======
We can not release all rmaps. If we do this, ->invalidate_page and
->invalidate_range_start can not find any spte using the host page,
that means, Accessed/Dirty for host page is missing tracked.
(missing call kvm_set_pfn_accessed and kvm_set_pfn_dirty properly.)

[https://lkml.org/lkml/2013/4/18/358]
======

Do you think this is a issue? What's your idea?

Thanks!

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