Re: [RFC] [VTD][patch 1/3] vt-d support for pci passthrough: kvm-vtd--kernel.patch

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Avi Kivity wrote:
> Anthony Liguori wrote:
>
>   
>>> What should be done for unmodified guest where there is no PV driver in
>>> the guest?  Would a call to mlock() from
>>> qemu/hw/pci-passthrough.c/add_pci_passthrough_device() a reasonable
>>> thing to do?
>>>       
>>
>> Yup.  The idea is to ensure that the memory is always present, 
>> without necessarily taking a reference to it.  This allows for memory 
>> reclaiming which should allow for things like NUMA page migration.  
>> We can't swap of course but that doesn't mean reclaimation isn't useful.
>>   
>
> I don't think we can do page migration with VT-d.  You need to be able 
> to detect whether the page has been changed by dma after you've copied 
> it but before you changed the pte, but VT-d doesn't allow that AFAICT.

Hrm, I would have to look at the VT-d but I suspect you're right.  
That's unfortunate.

That means mlock() isn't sufficient.  It also means that the VMAs can't 
be updated while the guest is running.  Is there any way to lock a vma 
region such that things like madvise/mmap(MAP_FIXED) will always fail?

Regards,

Anthony Liguori



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