Re: [PATCH v10 09/10] virtio-iommu: Set supported page size mask

[Peter Xu <peterx@redhat.com>]

On Thu, Oct 22, 2020 at 06:39:37PM +0200, Jean-Philippe Brucker wrote:
> So what I'd like to do for next version:
> 
> * Set qemu_real_host_page_mask as the default page mask, instead of the
>   rather arbitrary TARGET_PAGE_MASK.

Oh, I thought TARGET_PAGE_MASK was intended - kernel committ 39b3b3c9cac1
("iommu/virtio: Reject IOMMU page granule larger than PAGE_SIZE", 2020-03-27)
explicitly introduced a check that virtio-iommu kernel driver will fail
directly if this psize is bigger than PAGE_SIZE in the guest.  So it sounds
reasonable to have the default value as PAGE_SIZE (if it's the same as
TARGET_PAGE_SIZE in QEMU, which seems true?).

For example, I'm thinking whether qemu_real_host_page_mask could be bigger than
PAGE_SIZE in the guest in some environments, then it seems virtio-iommu won't
boot anymore without assigned devices, because that extra check above will
always fail.

>   Otherwise we cannot hotplug assigned
>   devices on a 64kB host, since TARGET_PAGE_MASK is pretty much always
>   4kB.
> 
> * Disallow changing the page size. It's simpler and works in
>   practice if we default to qemu_real_host_page_mask.
> 
> * For non-hotplug devices, allow changing the rest of the mask. For
>   hotplug devices, only warn about it.

Could I ask what's "the rest of the mask"?  On the driver side, I see that
viommu_domain_finalise() will pick the largest supported page size to use, if
so then we seem to be quite restricted on what page size we can use.

I'm also a bit curious about what scenario we plan to support in this initial
version, especially for ARM.  For x86, I think it's probably always 4k
everywhere so it's fairly simple.  Know little on ARM side...

Thanks,

-- 
Peter Xu