Re: [PATCH RFC 07/19] iommufd/vfio-compat: Dirty tracking IOCTLs compatibility

Joao Martins
2022-04-29T14:54:08Z

On 4/29/22 15:36, Jason Gunthorpe wrote:
> On Fri, Apr 29, 2022 at 03:27:00PM +0100, Joao Martins wrote:
>
>>> We've made a qemu patch to allow qemu to be happy if dirty tracking is
>>> not supported in the vfio container for migration, which is part of
>>> the v2 enablement series. That seems like the better direction.
>>>
>> So in my auditing/testing, the listener callbacks are called but the dirty ioctls
>> return an error at start, and bails out early on sync. I suppose migration
>> won't really work, as no pages aren't set and what not but it could
>> cope with no-dirty-tracking support. So by 'making qemu happy' is this mainly
>> cleaning out the constant error messages you get and not even attempt
>> migration by introducing a migration blocker early on ... should it fetch
>> no migration capability?
>
> It really just means pre-copy doesn't work and we can skip it, though
> I'm not sure exactly what the qemu patch ended up doing.. I think it
> will be posted by Monday
>
Ha, or that :D i.e.

Why bother checking if there's dirty pages periodically when we can just do at the
beginning, and at the end when we pause the guest(and DMA). Maybe it prevents a whole
bunch of copying in the interim, and this patch of yours might be a improvement.