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

[Eric Auger]

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?

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
>