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 > <jean-phili...@linaro.org> 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 > > > <jean-phili...@linaro.org> 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