On Wed, Sep 09, 2020 at 04:17:31PM +0800, Jason Wang wrote:
> After Linux kernel commit 61363c1474b1 ("iommu/vt-d: Enable ATS only
> if the device uses page aligned address."), ATS will be only enabled
> if device advertises a page aligned request.
>
> Unfortunately, vhost-net is the only user and we don't advertise the
> aligned request capability in the past since both vhost IOTLB and
> address_space_get_iotlb_entry() can support non page aligned request.
>
> Though it's not clear that if the above kernel commit makes
> sense. Let's advertise a page aligned ATS here to make vhost device
> IOTLB work with Intel IOMMU again.
IIUC the kernel commit should be needed because the VT-d Page Request
Descriptor used the rest bits of the address for other use (bits 11-0), so
logically an unaligned address can be mis-recognized with special meanings.
I'd guess some other archs (with its own IOMMU) might support unaligned
addresses and has different layout of page request descriptor, but not vt-d.
>
> Note that in the future we may extend pcie_ats_init() to accept
> parameters like queue depth and page alignment.
>
> Cc: qemu-sta...@nongnu.org
> Signed-off-by: Jason Wang <jasow...@redhat.com>
Reviewed-by: Peter Xu <pet...@redhat.com>
Maybe it would be good too that vhost provides real 4k-aligned addresses (in
vhost_iotlb_miss)? My understanding is that PCI_ATS_CAP_PAGE_ALIGNED will be
more compatible than without the bit set. E.g., so far vt-d emulation always
cut the address with 4k no matter what iova was passed in. However not sure
whether this will stop working with some new vIOMMUs joining.
Thanks,
--
Peter Xu
