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

[Jean-Philippe Brucker]

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);

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?

Thanks,
Jean