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

From: Peter Xu
Subject: Re: [PATCH v10 09/10] virtio-iommu: Set supported page size mask
Date: Fri, 23 Oct 2020 12:47:02 -0400

On Fri, Oct 23, 2020 at 09:48:58AM +0200, Jean-Philippe Brucker wrote:
> Arm CPUs and SMMU support 4k, 16k and 64k page sizes. I don't think 16k is
> used anywhere but some distributions chose 64k (RHEL, I think?), others
> 4k, so we need to support both.
> Unfortunately as noted above host64k-guest4k is not possible without
> adding a negotiation mechanism to virtio-iommu, host VFIO and IOMMU
> driver.

I see.  Then it seems we would still need to support host4k-guest64k.

Maybe for assigned case, we can simply AND all the psize_masks of all the vfio
containers that supported to replace the default psize mask (TARGET_PAGE_SIZE)
without caring about whether it's shrinking or not?  Note that current patch
only update config.psize_mask to the new one, but I think we need to calculate
the subset of all containers rather than a simply update.  Then with the help
of 39b3b3c9cac1 imho we'll gracefully fail the probe if the psize is not
suitable anyway, e.g., host64k-guest4k.


Peter Xu

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