Re: [PATCH 0/2] vfio/type1/pci: IOMMU PFNMAP invalidation

[Peter Xu]

On Thu, May 14, 2020 at 10:51:46AM -0600, Alex Williamson wrote:
> This is a follow-on series to "vfio-pci: Block user access to disabled
> device MMIO"[1], which extends user access blocking of disabled MMIO
> ranges to include unmapping the ranges from the IOMMU.  The first patch
> adds an invalidation callback path, allowing vfio bus drivers to signal
> the IOMMU backend to unmap ranges with vma level granularity.  This
> signaling is done both when the MMIO range becomes inaccessible due to
> memory disabling, as well as when a vma is closed, making up for the
> lack of tracking or pinning for non-page backed vmas.  The second
> patch adds registration and testing interfaces such that the IOMMU
> backend driver can test whether a given PFNMAP vma is provided by a
> vfio bus driver supporting invalidation.  We can then implement more
> restricted semantics to only allow PFNMAP DMA mappings when we have
> such support, which becomes the new default.

Hi, Alex,

IIUC we'll directly tearing down the IOMMU page table without telling the
userspace for those PFNMAP pages.  I'm thinking whether there be any side
effect on the userspace side when userspace cached these mapping information
somehow.  E.g., is there a way for userspace to know this event?

Currently, QEMU VT-d will maintain all the IOVA mappings for each assigned
device when used with vfio-pci.  In this case, QEMU will probably need to
depend some invalidations sent from the guest (either userspace or kernel)
device drivers to invalidate such IOVA mappings after they got removed from the
hardware IOMMU page table underneath.  I haven't thought deeper on what would
happen if the vIOMMU has got an inconsistent mapping of the real device.

Thanks,

-- 
Peter Xu