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

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Date:	Sun, 09 Jun 2013 17:08:34 +0800
From:	Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:	Gleb Natapov <gleb@...hat.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 06/09/2013 04:59 PM, Gleb Natapov wrote:
> 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.

Fine to me. I do not have strong opinion on that, please apply your patch instead. ;)



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