linux-kernel - Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

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Message-ID: <4ACD0DEE.80404@redhat.com>
Date:	Wed, 07 Oct 2009 23:53:50 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	kurt.hackel@...cle.com, the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Glauber de Oliveira Costa <gcosta@...hat.com>,
	Keir Fraser <keir.fraser@...citrix.com>,
	Zach Brown <zach.brown@...cle.com>,
	Chris Mason <chris.mason@...cle.com>
Subject: Re: [Xen-devel] Re: [PATCH 3/5] x86/pvclock: add vsyscall implementation

On 10/07/2009 11:51 PM, Jeremy Fitzhardinge wrote:
> On 10/07/09 14:37, Avi Kivity wrote:
>    
>> If the guest does a RMW on the version, but the host does not (copying
>> it from somewhere else), then the guest RMW can be lost.
>>
>> Looking at the code, that's what kvm does:
>>
>>      vcpu->hv_clock.version += 2;
>>
>>      shared_kaddr = kmap_atomic(vcpu->time_page, KM_USER0);
>>
>>      memcpy(shared_kaddr + vcpu->time_offset,&vcpu->hv_clock,
>>             sizeof(vcpu->hv_clock));
>>
>> so a guest-side ++version can be lost.
>>      
> I see, yes.  The Xen code explicitly reads back the guest version and
> increments that (I realize now that's what you meant by guest-private
> version).  If you were to have a second version number it would have to
> be separated as well to avoid being overwritten by the hypervisor.
>    

Yes.  We have the space since a cacheline is 64 bytes (minimum) vs 32 
bytes of pvclock data.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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