Re: [PATCH v3 0/4] virtio-iommu: Support VIRTIO_IOMMU_F_BYPASS_CONFIG

[Cornelia Huck]

On Tue, Feb 15 2022, Jean-Philippe Brucker <jean-philippe@linaro.org> wrote:

> On Tue, Feb 15, 2022 at 10:16:40AM +0100, Eric Auger wrote:
>> Hi Connie,
>> 
>> On 2/14/22 6:34 PM, Cornelia Huck wrote:
>> > On Mon, Feb 14 2022, Jean-Philippe Brucker <jean-philippe@linaro.org> 
>> > wrote:
>> >
>> >> Replace the VIRTIO_IOMMU_F_BYPASS feature with
>> >> VIRTIO_IOMMU_F_BYPASS_CONFIG, which enables a config space bit to switch
>> >> global bypass on and off.
>> >>
>> >> Add a boot-bypass option, which defaults to 'on' to be in line with
>> >> other vIOMMUs and to allow running firmware/bootloader that are unaware
>> >> of the IOMMU. x86 doesn't need a workaround to boot with virtio-iommu
>> >> anymore.
>> >>
>> >> Since v2 [1]:
>> >> * Added the new bypass bits to the migration stream.
>> >>   As discussed on the v2 thread, we assume that cross-version
>> >>   compatibility is not required for live migration at the moment, so we
>> >>   only increase the version number. Patch 2 says: "We add the bypass
>> >>   field to the migration stream without introducing subsections, based
>> >>   on the assumption that this virtio-iommu device isn't being used in
>> >>   production enough to require cross-version migration at the moment
>> >>   (all previous version required workarounds since they didn't support
>> >>   ACPI and boot-bypass)."
>> >>
>> >> [1] 
>> >> https://lore.kernel.org/qemu-devel/20220127142940.671333-1-jean-philippe@linaro.org/
>> > One thing that we could do to avoid surprises in the unlikely case that
>> > somebody has a virtio-iommu device and wants to migrate to an older
>> > machine version is to add a migration blocker for the virtio-iommu
>> > device for all compat machines for versions 6.2 or older (i.e. only 7.0
>> > or newer machine types can have a migratable virtio-iommu device
>> > starting with QEMU 7.0.) Not too complicated to implement, but I'm not
>> > sure whether we'd add too much code to prevent something very unlikely
>> > to happen anyway. I would not insist on it :)
>> As nobody has shout and we are not aware of anybody using the device in
>> production mode yet due to the missing boot bypass feature this series
>> brings, I would be personally in favour of leaving things as is. Now, up
>> to Jean if he wants to go and implement your suggestion.
>
> I agree, it seems too unlikely that someone would want to migrate it back
> to 6.2 where it wasn't really useable except for experiments

Fair enough. Let's make this an

Acked-by: Cornelia Huck <cohuck@redhat.com>

(for the series)