Re: [Qemu-devel] [RFC PATCH v3 3/3] VFIO Type1 IOMMU change: to support with iommu and without iommu

[Jike Song]

On 05/13/2016 04:12 AM, Neo Jia wrote:
> On Thu, May 12, 2016 at 01:05:52PM -0600, Alex Williamson wrote:
>>
>> If you're trying to equate the scale of what we need to track vs what
>> type1 currently tracks, they're significantly different.  Possible
>> things we need to track include the pfn, the iova, and possibly a
>> reference count or some sort of pinned page map.  In the pin-all model
>> we can assume that every page is pinned on map and unpinned on unmap,
>> so a reference count or map is unnecessary.  We can also assume that we
>> can always regenerate the pfn with get_user_pages() from the vaddr, so
>> we don't need to track that.  
> 
> Hi Alex,
> 
> Thanks for pointing this out, we will not track those in our next rev and
> get_user_pages will be used from the vaddr as you suggested to handle the
> single VM with both passthru + mediated device case.
>

Just a gut feeling:

Calling GUP every time for a particular vaddr, means locking mm->mmap_sem
every time for a particular process. If the VM has dozens of VCPU, which
is not rare, the semaphore is likely to be the bottleneck.


--
Thanks,
Jike