Re: [Qemu-devel] [PATCH v3 00/12] intel-iommu: nested vIOMMU, cleanups, bug fixes

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, May 17, 2018 at 04:59:15PM +0800, Peter Xu wrote:
> (Hello, Jintack, Feel free to test this branch again against your scp
>  error case when you got free time)
> 
> I rewrote some of the patches in V3.  Major changes:
> 
> - Dropped mergable interval tree, instead introduced IOVA tree, which
>   is even simpler.
> 
> - Fix the scp error issue that Jintack reported.  Please see patches
>   for detailed information.  That's the major reason to rewrite a few
>   of the patches.  We use replay for domain flushes are possibly
>   incorrect in the past.  The thing is that IOMMU replay has an
>   "definition" that "we should only send MAP when new page detected",
>   while for shadow page syncing we actually need something else than
>   that.  So in this version I started to use a new
>   vtd_sync_shadow_page_table() helper to do the page sync.
> 
> - Some other refines after the refactoring.
> 
> I'll add unit test for the IOVA tree after this series merged to make
> sure we won't switch to another new tree implementaion...
> 
> The element size in the new IOVA tree should be around
> sizeof(GTreeNode + IOMMUTLBEntry) ~= (5*8+4*8) = 72 bytes.  So the
> worst case usage ratio would be 72/4K=2%, which still seems acceptable
> (it means 8G L2 guest will use 8G*2%=160MB as metadata to maintain the
> mapping in QEMU).
> 
> I did explicit test with scp this time, copying 1G sized file for >10
> times on each of the following case:
> 
> - L1 guest, with vIOMMU and with assigned device
> - L2 guest, without vIOMMU and with assigned device
> - L2 guest, with vIOMMU (so 3-layer nested IOMMU) and with assigned device
> 
> Please review.  Thanks,
> 
> (Below are old content from previous cover letter)
> 
> ==========================
> 
> v2:
> - fix patchew code style warnings
> - interval tree: postpone malloc when inserting; simplify node remove
>   a bit where proper [Jason]
> - fix up comment and commit message for iommu lock patch [Kevin]
> - protect context cache too using the iommu lock [Kevin, Jason]
> - add vast comment in patch 8 to explain the modify-PTE problem
>   [Jason, Kevin]
> 
> Online repo:
> 
>   https://github.com/xzpeter/qemu/tree/fix-vtd-dma
> 
> This series fixes several major problems that current code has:
> 
> - Issue 1: when getting very big PSI UNMAP invalidations, the current
>   code is buggy in that we might skip the notification while actually
>   we should always send that notification.

security issue

> - Issue 2: IOTLB is not thread safe, while block dataplane can be
>   accessing and updating it in parallel.

security issue

> - Issue 3: For devices that only registered with UNMAP-only notifiers,
>   we don't really need to do page walking for PSIs, we can directly
>   deliver the notification down.  For example, vhost.

optimization

> - Issue 4: unsafe window for MAP notified devices like vfio-pci (and
>   in the future, vDPA as well).  The problem is that, now for domain
>   invalidations we do this to make sure the shadow page tables are
>   correctly synced:
> 
>   1. unmap the whole address space
>   2. replay the whole address space, map existing pages
> 
>   However during step 1 and 2 there will be a very tiny window (it can
>   be as big as 3ms) that the shadow page table is either invalid or
>   incomplete (since we're rebuilding it up).  That's fatal error since
>   devices never know that happending and it's still possible to DMA to
>   memories.

correctness but not a security issue

> Patch 1 fixes issue 1.  I put it at the first since it's picked from
> an old post.
> 
> Patch 2 is a cleanup to remove useless IntelIOMMUNotifierNode struct.
> 
> Patch 3 fixes issue 2.
> 
> Patch 4 fixes issue 3.
> 
> Patch 5-9 fix issue 4.  Here a very simple interval tree is
> implemented based on Gtree.  It's different with general interval tree
> in that it does not allow user to pass in private data (e.g.,
> translated addresses).  However that benefits us that then we can
> merge adjacent interval leaves so that hopefully we won't consume much
> memory even if the mappings are a lot (that happens for nested virt -
> when mapping the whole L2 guest RAM range, it can be at least in GBs).
> 
> Patch 10 is another big cleanup only can work after patch 9.


So 1-2 are needed on stable. 1-9 would be nice to have
there too, even though they are big and it looks risky.

> Tests:
> 
> - device assignments to L1, even L2 guests.  With this series applied
>   (and the kernel IOMMU patches: https://lkml.org/lkml/2018/4/18/5),
>   we can even nest vIOMMU now, e.g., we can specify vIOMMU in L2 guest
>   with assigned devices and things will work.  We can't before.
> 
> - vhost smoke test for regression.
> 
> Please review.  Thanks,
> 
> Peter Xu (12):
>   intel-iommu: send PSI always even if across PDEs
>   intel-iommu: remove IntelIOMMUNotifierNode
>   intel-iommu: add iommu lock
>   intel-iommu: only do page walk for MAP notifiers
>   intel-iommu: introduce vtd_page_walk_info
>   intel-iommu: pass in address space when page walk
>   intel-iommu: trace domain id during page walk
>   util: implement simple iova tree
>   intel-iommu: maintain per-device iova ranges
>   intel-iommu: simplify page walk logic
>   intel-iommu: new vtd_sync_shadow_page_table_range
>   intel-iommu: new sync_shadow_page_table
> 
>  include/hw/i386/intel_iommu.h |  19 +-
>  include/qemu/iova-tree.h      | 134 ++++++++++++
>  hw/i386/intel_iommu.c         | 381 +++++++++++++++++++++++++---------
>  util/iova-tree.c              | 114 ++++++++++
>  MAINTAINERS                   |   6 +
>  hw/i386/trace-events          |   5 +-
>  util/Makefile.objs            |   1 +
>  7 files changed, 556 insertions(+), 104 deletions(-)
>  create mode 100644 include/qemu/iova-tree.h
>  create mode 100644 util/iova-tree.c
> 
> -- 
> 2.17.0