Re: Re: [PATCH v8 10/10] Documentation: Add documentation for VDUSE

[Stefan Hajnoczi]

On Tue, Jun 29, 2021 at 01:43:11PM +0800, Yongji Xie wrote:
> On Mon, Jun 28, 2021 at 9:02 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> wrote:
> > On Tue, Jun 15, 2021 at 10:13:31PM +0800, Xie Yongji wrote:
> > > +     static void *iova_to_va(int dev_fd, uint64_t iova, uint64_t *len)
> > > +     {
> > > +             int fd;
> > > +             void *addr;
> > > +             size_t size;
> > > +             struct vduse_iotlb_entry entry;
> > > +
> > > +             entry.start = iova;
> > > +             entry.last = iova + 1;
> >
> > Why +1?
> >
> > I expected the request to include *len so that VDUSE can create a bounce
> > buffer for the full iova range, if necessary.
> >
> 
> The function is used to translate iova to va. And the *len is not
> specified by the caller. Instead, it's used to tell the caller the
> length of the contiguous iova region from the specified iova. And the
> ioctl VDUSE_IOTLB_GET_FD will get the file descriptor to the first
> overlapped iova region. So using iova + 1 should be enough here.

Does the entry.last field have any purpose with VDUSE_IOTLB_GET_FD? I
wonder why userspace needs to assign a value at all if it's always +1.

> 
> > > +             fd = ioctl(dev_fd, VDUSE_IOTLB_GET_FD, &entry);
> > > +             if (fd < 0)
> > > +                     return NULL;
> > > +
> > > +             size = entry.last - entry.start + 1;
> > > +             *len = entry.last - iova + 1;
> > > +             addr = mmap(0, size, perm_to_prot(entry.perm), MAP_SHARED,
> > > +                         fd, entry.offset);
> > > +             close(fd);
> > > +             if (addr == MAP_FAILED)
> > > +                     return NULL;
> > > +
> > > +             /* do something to cache this iova region */
> >
> > How is userspace expected to manage iotlb mmaps? When should munmap(2)
> > be called?
> >
> 
> The simple way is using a list to store the iotlb mappings. And we
> should call the munmap(2) for the old mappings when VDUSE_UPDATE_IOTLB
> or VDUSE_STOP_DATAPLANE message is received.

Thanks for explaining. It would be helpful to have a description of
IOTLB operation in this document.

> > Should userspace expect VDUSE_IOTLB_GET_FD to return a full chunk of
> > guest RAM (e.g. multiple gigabytes) that can be cached permanently or
> > will it return just enough pages to cover [start, last)?
> >
> 
> It should return one iotlb mapping that covers [start, last). In
> vhost-vdpa cases, it might be a full chunk of guest RAM. In
> virtio-vdpa cases, it might be the whole bounce buffer or one coherent
> mapping (produced by dma_alloc_coherent()).

Great, thanks. Adding something about this to the documentation would
help others implementing VDUSE devices or libraries.

> > > +
> > > +             return addr + iova - entry.start;
> > > +     }
> > > +
> > > +- VDUSE_DEV_GET_FEATURES: Get the negotiated features
> >
> > Are these VIRTIO feature bits? Please explain how feature negotiation
> > works. There must be a way for userspace to report the device's
> > supported feature bits to the kernel.
> >
> 
> Yes, these are VIRTIO feature bits. Userspace will specify the
> device's supported feature bits when creating a new VDUSE device with
> ioctl(VDUSE_CREATE_DEV).

Can the VDUSE device influence feature bit negotiation? For example, if
the VDUSE virtio-blk device does not implement discard/write-zeroes, how
does QEMU or the guest find out about this?

> > > +- VDUSE_DEV_UPDATE_CONFIG: Update the configuration space and inject a config interrupt
> >
> > Does this mean the contents of the configuration space are cached by
> > VDUSE?
> 
> Yes, but the kernel will also store the same contents.
> 
> > The downside is that the userspace code cannot generate the
> > contents on demand. Most devices doin't need to generate the contents
> > on demand, so I think this is okay but I had expected a different
> > interface:
> >
> > kernel->userspace VDUSE_DEV_GET_CONFIG
> > userspace->kernel VDUSE_DEV_INJECT_CONFIG_IRQ
> >
> 
> The problem is how to handle the failure of VDUSE_DEV_GET_CONFIG. We
> will need lots of modification of virtio codes to support that. So to
> make it simple, we choose this way:
> 
> userspace -> kernel VDUSE_DEV_SET_CONFIG
> userspace -> kernel VDUSE_DEV_INJECT_CONFIG_IRQ
> 
> > I think you can leave it the way it is, but I wanted to mention this in
> > case someone thinks it's important to support generating the contents of
> > the configuration space on demand.
> >
> 
> Sorry, I didn't get you here. Can't VDUSE_DEV_SET_CONFIG and
> VDUSE_DEV_INJECT_CONFIG_IRQ achieve that?

If the contents of the configuration space change continuously, then the
VDUSE_DEV_SET_CONFIG approach is inefficient and might have race
conditions. For example, imagine a device where the driver can read a
timer from the configuration space. I think the VIRTIO device model
allows that although I'm not aware of any devices that do something like
it today. The problem is that VDUSE_DEV_SET_CONFIG would have to be
called frequently to keep the timer value updated even though the guest
driver probably isn't accessing it.

What's worse is that there might be race conditions where other
driver->device operations are supposed to update the configuration space
but VDUSE_DEV_SET_CONFIG means that the VDUSE kernel code is caching an
outdated copy.

Again, I don't think it's a problem for existing devices in the VIRTIO
specification. But I'm not 100% sure and future devices might require
what I've described, so the VDUSE_DEV_SET_CONFIG interface could become
a problem.

Stefan

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