linux-kernel - Re: [PATCH] KVM: x86: fix missed memory synchronization when patch hypercall

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Date:	Sun, 9 Jun 2013 11:59:38 +0300
From:	Gleb Natapov <gleb@...hat.com>
To:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
Cc:	Paolo Bonzini <pbonzini@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH] KVM: x86: fix missed memory synchronization when patch
 hypercall

On Sun, Jun 09, 2013 at 04:56:42PM +0800, Xiao Guangrong wrote:
> On 06/09/2013 04:45 PM, Gleb Natapov wrote:
> > On Sat, Jun 08, 2013 at 11:15:37AM +0800, Xiao Guangrong wrote:
> >> From: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> >>
> >> Currently, memory synchronization is missed in emulator_fix_hypercall,
> >> please see the commit 758ccc89b83
> >> (KVM: x86: drop calling kvm_mmu_zap_all in emulator_fix_hypercall)
> >>
> >> This patch fixes it by introducing kvm_vcpus_hang_on_page_start() and
> >> kvm_vcpus_hang_on_page_end which unmap the patched page from guest
> >> and use kvm_flush_remote_tlbs() as the serializing instruction to
> >> ensure the memory coherence
> >> [ The SDM said that INVEPT, INVVPID and MOV (to control register, with
> >>   the exception of MOV CR8) are the serializing instructions. ]
> >>
> >> The mmu-lock is held during host patches the page so that it stops vcpus
> >> to fix its further page fault
> >>
> > I have a patch to implement is much simple and in generic way, not
> > relying on MMU internals.
> 
> I have considered this way but it seems not simple - it needs a new type of
> request and it forces all vcpus to hang when host is patching the page.
> 
> My approach is just reusing the mmu code and requires vcpus to hang only when
> the patched page is bing accessed.

That's very rare, no point to optimize this code path.

--
			Gleb.
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