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Re: [Qemu-devel] [PATCH v5 02/18] vfio: introduce vfio_get_vaddr()

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH v5 02/18] vfio: introduce vfio_get_vaddr()
Date: Wed, 25 Jan 2017 18:40:56 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

On 25/01/2017 18:36, Alex Williamson wrote:
>> You probably should also put a comment about why VFIO does *not* need to
>> keep a reference between vfio_dma_map and vfio_dma_unmap (which doesn't
>> sound easy to do either).  Would any well-behaved guest invalidate the
>> IOMMU page tables before a memory hot-unplug?
> Hmm, we do take a reference in vfio_listener_region_add(), but this is
> of course to the iommu region not to the RAM region we're translating.
> In the non-vIOMMU case we would be holding a reference to the memory
> region backing a DMA mapping.  I would expect a well behaved guest to
> evacuate DMA mappings targeting a hotplug memory region before it gets
> ejected, but how much do we want to rely on well behaved guests.

It depends of what happens if they aren't.  I think it's fine (see other
message), but taking a reference for each mapping entry isn't so easy
because the unmap case doesn't know the old memory region.


> Perhaps we should be taking a reference for each mapping entry, though
> this makes Peter's plans to invalidate the entire address space much
> more difficult.  Thanks,

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