Re: [Qemu-devel] Re: [PATCH-RFC 0/3] qemu: memory barriers in virtio

[Avi Kivity <avi@redhat.com>]

On 12/22/2009 06:25 PM, Paul Brook wrote:
> On Tuesday 22 December 2009, Anthony Liguori wrote:
>    
>> On 12/22/2009 05:26 AM, Michael S. Tsirkin wrote:
>>      
>>> On Tue, Dec 08, 2009 at 06:18:18PM +0200, Michael S. Tsirkin wrote:
>>>        
>>>> The following fixes a class of long-standing bugs in qemu:
>>>> when kvm is enabled, guest might access device structures
>>>> in memory while they are updated by qemu on another CPU.
>>>> In this scenario, memory barriers are necessary to prevent
>>>> host CPU from reordering memory accesses, which might confuse
>>>> the guest.
>>>>
>>>> This patch only fixes virtio, but other emulated devices
>>>> might have a similar bug. They'll need to be discovered
>>>> and addressed case by case.
>>>>          
> Real devices generally aren't cache coherent, so I'd expect problems to be
> rare. I guess theoretically you may need barriers around the MMIO/IO port
> handlers, though in practice the KVM context switch probably provides this
> anyway.
>    

We're not guaranteed to have a context switch.  One thread can update 
the ring while another consumes it.

-- 
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