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

From: Jean-Philippe Brucker
Subject: Re: [PATCH v10 09/10] virtio-iommu: Set supported page size mask
Date: Tue, 27 Oct 2020 18:38:40 +0100

On Fri, Oct 23, 2020 at 12:47:02PM -0400, Peter Xu wrote:
> 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.

Yes I think an AND is the right operation. We'll return an error if the
resulting mask is 0. Then for hotplug, I think I'll keep the current "best
effort" code from this patch. If necessary we could later add a parameter
to set a default mask and guarantee hotplug success.


> Then with the help
> of 39b3b3c9cac1 imho we'll gracefully fail the probe if the psize is not
> suitable anyway, e.g., host64k-guest4k.
> Thanks,
> -- 
> Peter Xu

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