Re: [PATCH v7 3/5] virtio-iommu: Call iommu notifier for attach/detach

[Jean-Philippe Brucker]

On Wed, Mar 18, 2020 at 03:47:44PM +0530, Bharat Bhushan wrote:
> Hi Jean,
> 
> On Tue, Mar 17, 2020 at 9:29 PM Jean-Philippe Brucker
> <address@hidden> wrote:
> >
> > On Tue, Mar 17, 2020 at 02:46:55PM +0530, Bharat Bhushan wrote:
> > > Hi Jean,
> > >
> > > On Tue, Mar 17, 2020 at 2:23 PM Jean-Philippe Brucker
> > > <address@hidden> wrote:
> > > >
> > > > On Tue, Mar 17, 2020 at 12:40:39PM +0530, Bharat Bhushan wrote:
> > > > > Hi Jean,
> > > > >
> > > > > On Mon, Mar 16, 2020 at 3:41 PM Jean-Philippe Brucker
> > > > > <address@hidden> wrote:
> > > > > >
> > > > > > Hi Bharat,
> > > > > >
> > > > > > Could you Cc me on your next posting?  Unfortunately I don't have 
> > > > > > much
> > > > > > hardware for testing this at the moment, but I might be able to 
> > > > > > help a
> > > > > > little on the review.
> > > > > >
> > > > > > On Mon, Mar 16, 2020 at 02:40:00PM +0530, Bharat Bhushan wrote:
> > > > > > > > >>> First issue is: your guest can use 4K page and your host 
> > > > > > > > >>> can use 64KB
> > > > > > > > >>> pages. In that case VFIO_DMA_MAP will fail with -EINVAL. We 
> > > > > > > > >>> must devise
> > > > > > > > >>> a way to pass the host settings to the VIRTIO-IOMMU device.
> > > > > > > > >>>
> > > > > > > > >>> Even with 64KB pages, it did not work for me. I have 
> > > > > > > > >>> obviously not the
> > > > > > > > >>> storm of VFIO_DMA_MAP failures but I have some, most 
> > > > > > > > >>> probably due to
> > > > > > > > >>> some wrong notifications somewhere. I will try to 
> > > > > > > > >>> investigate on my side.
> > > > > > > > >>>
> > > > > > > > >>> Did you test with VFIO on your side?
> > > > > > > > >>
> > > > > > > > >> I did not tried with different page sizes, only tested with 
> > > > > > > > >> 4K page size.
> > > > > > > > >>
> > > > > > > > >> Yes it works, I tested with two n/w device assigned to VM, 
> > > > > > > > >> both interfaces works
> > > > > > > > >>
> > > > > > > > >> First I will try with 64k page size.
> > > > > > > > >
> > > > > > > > > 64K page size does not work for me as well,
> > > > > > > > >
> > > > > > > > > I think we are not passing correct page_size_mask here
> > > > > > > > > (config.page_size_mask is set to TARGET_PAGE_MASK ( which is
> > > > > > > > > 0xfffffffffffff000))
> > > > > > > > I guess you mean with guest using 4K and host using 64K.
> > > > > > > > >
> > > > > > > > > We need to set this correctly as per host page size, correct?
> > > > > > > > Yes that's correct. We need to put in place a control path to 
> > > > > > > > retrieve
> > > > > > > > the page settings on host through VFIO to inform the 
> > > > > > > > virtio-iommu device.
> > > > > > > >
> > > > > > > > Besides this issue, did you try with 64kB on host and guest?
> > > > > > >
> > > > > > > I tried Followings
> > > > > > >   - 4k host and 4k guest  - it works with v7 version
> > > > > > >   - 64k host and 64k guest - it does not work with v7
> > > > > > >     hard-coded config.page_size_mask to 0xffffffffffff0000 and it 
> > > > > > > works
> > > > > >
> > > > > > You might get this from the iova_pgsize bitmap returned by
> > > > > > VFIO_IOMMU_GET_INFO. The virtio config.page_size_mask is global so 
> > > > > > there
> > > > > > is the usual problem of aggregating consistent properties, but I'm
> > > > > > guessing using the host page size as a granule here is safe enough.
> > > > > >
> > > > > > If it is a problem, we can add a PROBE property for page size mask,
> > > > > > allowing to define per-endpoint page masks. I have kernel patches
> > > > > > somewhere to do just that.
> > > > >
> > > > > I do not see we need page size mask per endpoint.
> > > > >
> > > > > While I am trying to understand what "page-size-mask" guest will work 
> > > > > with
> > > > >
> > > > > - 4K page size host and 4k page size guest
> > > > >   config.page_size_mask = 0xffffffffffff000 will work
> > > > >
> > > > > - 64K page size host and 64k page size guest
> > > > >   config.page_size_mask = 0xfffffffffff0000 will work
> > > > >
> > > > > - 64K page size host and 4k page size guest
> > > > >    1) config.page_size_mask = 0xffffffffffff000 will also not work as
> > > > > VFIO in host expect iova and size to be aligned to 64k (PAGE_SIZE in
> > > > > host)
> > > > >    2) config.page_size_mask = 0xfffffffffff0000 will not work, iova
> > > > > initialization (in guest) expect minimum page-size supported by h/w to
> > > > > be equal to 4k (PAGE_SIZE in guest)
> > > > >        Should we look to relax this in iova allocation code?
> > > >
> > > > Oh right, that's not great. Maybe the BUG_ON() can be removed, I'll ask 
> > > > on
> > > > the list.
> > >
> > > yes, the BUG_ON in iova_init.
> > > I tried with removing same and it worked, but not analyzed side effects.
> >
> > It might break the assumption from device drivers that mapping a page is
> > safe. For example they call alloc_page() followed by dma_map_page(). In
> > our situation dma-iommu.c will oblige and create one 64k mapping to one 4k
> > physical page. As a result the endpoint can access the neighbouring 60k of
> > memory.
> >
> > This isn't too terrible. After all, even when the page sizes match, device
> > drivers can call dma_map_single() on sub-page buffers, which will also let
> > the endpoint access a whole page. The solution, if you don't trust the
> > endpoint, is to use bounce buffers.
> >
> > But I suspect it's not as simple as removing the BUG_ON(), we'll need to
> > go over dma-iommu.c first. And it seems like assigning endpoints to guest
> > userspace won't work either in this config. In vfio_dma_do_map():
> >
> >         mask = ((uint64_t)1 << __ffs(vfio_pgsize_bitmap(iommu))) - 1;
> >
> >         WARN_ON(mask & PAGE_MASK);
> 
> Yes, Agree
> 
> >
> > If I read this correctly the WARN will trigger in a 4k guest under 64k
> > host, right?  So maybe we can just say that this config isn't supported,
> > unless it's an important use-case for virtio-iommu?
> 
> I sent v8 version of patch and with that guest and host with same page
> size should work.
> While i have not yet added analyzed how to mark 4k guest and 64k host
> as un-supported configuration, will analyze and send patch.

I don't think there is anything to do for QEMU, it's Linux that doesn't
support the configuration. We could add something like the attached patch,
in the virtio-iommu driver, to abort more gracefully than with a BUG_ON().

Thanks,
Jean

0001-iommu-virtio-Reject-IOMMU-page-granule-larger-than-P.patch
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