Re: [PATCH v2] virtio-iommu: Use qemu_real_host_page_mask as default page_size_mask

[Eric Auger <eric.auger@redhat.com>]

On 2/21/24 12:31, Jean-Philippe Brucker wrote:
> On Wed, Feb 21, 2024 at 11:41:57AM +0100, Eric Auger wrote:
>> Hi,
>>
>> On 2/13/24 13:00, Michael S. Tsirkin wrote:
>>> On Tue, Feb 13, 2024 at 12:24:22PM +0100, Eric Auger wrote:
>>>> Hi Michael,
>>>> On 2/13/24 12:09, Michael S. Tsirkin wrote:
>>>>> On Tue, Feb 13, 2024 at 11:32:13AM +0100, Eric Auger wrote:
>>>>>> Do you have an other concern?
>>>>> I also worry a bit about migrating between hosts with different
>>>>> page sizes. Not with kvm I am guessing but with tcg it does work I think?
>>>> I have never tried but is it a valid use case? Adding Peter in CC.
>>>>> Is this just for vfio and vdpa? Can we limit this to these setups
>>>>> maybe?
>>>> I am afraid we know the actual use case too later. If the VFIO device is
>>>> hotplugged we have started working with 4kB granule.
>>>>
>>>> The other way is to introduce a min_granule option as done for aw-bits.
>>>> But it is heavier.
>>>>
>>>> Thanks
>>>>
>>>> Eric
>>> Let's say, if you are changing the default then we definitely want
>>> a way to get the cmpatible behaviour for tcg.
>>> So the compat machinery should be user-accessible too and documented.
>> I guess I need to add a new option to guarantee the machine compat.
>>
>> I was thinking about an enum GranuleMode property taking the following
>> values, 4KB, 64KB, host
>> Jean, do you think there is a rationale offering something richer?
> 16KB seems to be gaining popularity, we should include that (I think it's
> the only granule supported by Apple IOMMU?). Hopefully that will be
> enough.
thank you for your prompt reply. I do agree.

nevertheless I am going to prototype passing the whole page size mask I
think. That could be useful to test some weird configs

Thanks

Eric
>
> Thanks,
> Jean
>
>> Obviously being able to set the exact page_size_mask + host mode would
>> be better but this does not really fit into any std property type.
>>
>> Thanks
>>
>> Eric