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

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Date:	Sun, 21 Apr 2013 12:24:31 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.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 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?


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