Re: [PATCH 0/9] migration: Drop/unexport migration_is_device() and migration_is_active()

[Joao Martins]

On 16/12/2024 09:46, Avihai Horon wrote:
> Hello,
> 
> This follows up on Peter's series [1] to simplify migration status API
> to a single migration_is_running() function.
> 
> Peter's series tried to drop migration_is_device() and
> migration_is_active(), however VFIO used them to check if dirty page
> tracking has been started in order to avoid errors in log sync, so they
> couldn't simply be dropped without some preliminary cleanups.
> 
> This series handles these preliminary cleanups and eventually drops
> migration_is_device() and unexports migration_is_active().
> 
> The series has been migration tested with the following:
> - VFIO device dirty tracking.
> - Legacy VFIO iommu dirty tracking.
> - vIOMMU + Legacy VFIO iommu dirty tracking (migration with vIOMMU is
>   currently blocked, so I used a patched QEMU to allow it).
>

vIOMMU on IOMMU HW doesn't suffer from the same problems of VF dirty tracking
where there's a aggregate limit into how much VFs can track in terms of IOVA
space. So we can lift some of those restrictions for IOMMU even right now
provided we implement the last remaining pre-requisite. I also have a much
smaller series for that sort of unblockage that I can give you a pointer.
Though, eventually the optimizations we will do for VF dirty tracking for vIOMMU
will apply to IOMMU HW too just so we minimize the amount of calls to get dirty
bits.

> I didn't test it with iommu DPT as I don't have access to such HW.
> Cedric, I remember you said that you have such HW, it would be very
> helpful if you could test it.
> 
I am starting to prep the unblocking vIOMMU for Qemu 10, so I can validate if
this series works as well -- but from what I have looked so far it should be all
OK. If it helps I have some pending series that lets you test emulated x86 IOMMU
DPT support (either on intel-iommu or amd-iommu) that can help you when you
don't have the hardware to test.

Regarding this series, since you are looking at the the dirty tracking 'status'
I'll comment here too as one of your original patches introduced it as it's
related to the use of migration_is_running(). And since you're looking at this
exact part of the code, might as well cover that too.