[virtio-dev] Re: [PATCH V2] docs: vhost-user: Add Xen specific memory mapping support

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Mar 07, 2023 at 11:13:36AM +0530, Viresh Kumar wrote:
> On 06-03-23, 10:34, Stefan Hajnoczi wrote:
> > On Mon, Mar 06, 2023 at 04:40:24PM +0530, Viresh Kumar wrote:
> > > +Xen mmap description
> > > +^^^^^^^^^^^^^^^^^^^^
> > > +
> > > ++-------+-------+
> > > +| flags | domid |
> > > ++-------+-------+
> > > +
> > > +:flags: 64-bit bit field
> > > +
> > > +- Bit 0 is set for Xen foreign memory memory mapping.
> > > +- Bit 1 is set for Xen grant memory memory mapping.
> > > +- Bit 2 is set if the back-end can directly map additional memory (like
> > > +  descriptor buffers or indirect descriptors, which aren't part of already
> > > +  shared memory regions) without the need of front-end sending an additional
> > > +  memory region first.
> > 
> > I don't understand what Bit 2 does. Can you rephrase this? It's unclear
> > to me how additional memory can be mapped without a memory region
> > (especially the fd) is sent?
> 
> I (somehow) assumed we will be able to use the same file descriptor
> that was shared for the virtqueues memory regions and yes I can see
> now why it wouldn't work or create problems.
> 
> And I need suggestion now on how to make this work.
> 
> With Xen grants, the front end receives grant address from the from
> guest kernel, they aren't physical addresses, kind of IOMMU stuff.
> 
> The back-end gets access for memory regions of the virtqueues alone
> initially.  When the back-end gets a request, it reads the descriptor
> and finds the buffer address, which isn't part of already shared
> regions. The same happens for descriptor addresses in case indirect
> descriptor feature is negotiated.
> 
> At this point I was thinking maybe the back-end can simply call the
> mmap/ioctl to map the memory, using the file descriptor used for the
> virtqueues.
> 
> How else can we make this work ? We also need to unmap/remove the
> memory region, as soon as the buffer is processed as the grant address
> won't be relevant for any subsequent request.
> 
> Should I use VHOST_USER_IOTLB_MSG for this ? I did look at it and I
> wasn't convinced if it was an exact fit. For example it says that a
> memory address reported with miss/access fail should be part of an
> already sent memory region, which isn't the case here.

VHOST_USER_IOTLB_MSG probably isn't necessary because address
translation is not required. It will also reduce performance by adding
extra communication.

Instead, you could change the 1 memory region : 1 mmap relationship that
existing non-Xen vhost-user back-end implementations have. In Xen
vhost-user back-ends, the memory region details (including the file
descriptor and Xen domain id) would be stashed away in back-end when the
front-end adds memory regions. No mmap would be performed upon
VHOST_USER_ADD_MEM_REG or VHOST_USER_SET_MEM_TABLE.

Whenever the back-end needs to do DMA, it looks up the memory region and
performs the mmap + Xen-specific calls:
- A long-lived mmap of the vring is set up when
  VHOST_USER_SET_VRING_ENABLE is received.
- Short-lived mmaps of the indirect descriptors and memory pointed to by
  the descriptors is set up by the virtqueue processing code.

Does this sound workable to you?

Stefan

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