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

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Message-ID: <5173F319.2040106@linux.vnet.ibm.com>
Date:	Sun, 21 Apr 2013 22:09:29 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Gleb Natapov <gleb@...hat.com>
CC:	mtosatti@...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 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!

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