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

[Eric Auger]

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.

Thanks

Eric

>
> Otherwise, I didn't spot problems in this series.
>