Re: [PATCH v4 18/21] drivers: hv: vmbus: replace deprecated mmap hook with mmap_prepare

["Lorenzo Stoakes (Oracle)" <ljs@kernel.org>]

On Mon, Mar 23, 2026 at 04:16:20AM +0000, Michael Kelley wrote:
> From: Lorenzo Stoakes (Oracle) <ljs@kernel.org> Sent: Friday, March 20, 2026 3:40 PM
> >
> > The f_op->mmap interface is deprecated, so update the vmbus driver to use
> > its successor, mmap_prepare.
> >
> > This updates all callbacks which referenced the function pointer
> > hv_mmap_ring_buffer to instead reference hv_mmap_prepare_ring_buffer,
> > utilising the newly introduced compat_set_desc_from_vma() and
> > __compat_vma_mmap() to be able to implement this change.
> >
> > The UIO HV generic driver is the only user of hv_create_ring_sysfs(),
> > which is the only function which references
> > vmbus_channel->mmap_prepare_ring_buffer which, in turn, is the only
> > external interface to hv_mmap_prepare_ring_buffer.
> >
> > This patch therefore updates this caller to use mmap_prepare instead,
> > which also previously used vm_iomap_memory(), so this change replaces it
> > with its mmap_prepare equivalent, mmap_action_simple_ioremap().
> >
> > Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
> > ---
> >  drivers/hv/hyperv_vmbus.h    |  4 ++--
> >  drivers/hv/vmbus_drv.c       | 31 +++++++++++++++++++------------
> >  drivers/uio/uio_hv_generic.c | 11 ++++++-----
> >  include/linux/hyperv.h       |  4 ++--
> >  4 files changed, 29 insertions(+), 21 deletions(-)
> >
>
> There are two mmap() code paths in the Hyper-V UIO code. One path is
> to mmap() the file descriptor for /dev/uio<n>, and the other is to mmap()
> the "ring" entry under /sys/devices/vmbus/devices/<uuid>. The former is
> done by uio_mmap(), and the latter by hv_uio_ring_mmap_prepare().
>
> I tested both these paths using a combination of two methods in a
> x86/x64 VM on Hyper-V:
>
> 1) Using the fcopy daemon, which maps the ring buffer for the primary
> channel and sends/receives messages with the Hyper-V host. This
> method tests only the 1st path because the fcopy daemon doesn't create
> any subchannels that would use the "ring" entry.
>
> 2) Using a custom-built test program. This program doesn't communicate
> with the Hyper-V host, but allows mostly verifying both code paths for the
> primary channel. As a sanity check, it verifies that the two mmaps are
> mapping the same memory, as expected.
>
> As such,
>
> Reviewed-by: Michael Kelley <mhklinux@outlook.com>
> Tested-by: Michael Kelley <mhklinux@outlook.com>

Perfect, thanks so much for this!

It is tricky for me to test these, beyond fairly exhaustive logical
confirmation of equivalence, so this is _hugely_ helpful.

>
> The most robust test would be to run DPDK networking against
> UIO, as it would communicate with the Hyper-V host and use
> multiple subchannels that resulting in mmap'ing the "ring"
> entry under /sys.
>
> @Long Li -- I'll leave it to your discretion as to whether you want
> to test DPDK against these mmap() changes.

Thanks in advance for taking a look on this also!

>
> I've noted one minor issue below.
>
> [snip]
>
> --- a/include/linux/hyperv.h
> +++ b/include/linux/hyperv.h
> @@ -1015,8 +1015,8 @@ struct vmbus_channel {
>  	/* The max size of a packet on this channel */
>  	u32 max_pkt_size;
>
> -	/* function to mmap ring buffer memory to the channel's sysfs ring attribute */
> -	int (*mmap_ring_buffer)(struct vmbus_channel *channel, struct vm_area_struct *vma);
> +	/* function to mmap_prepare ring buffer memory to the channel's sysfs ring attribute */
>
> Changing the comment from "mmap ring buffer" to "mmap_prepare ring buffer"
> produces awkward wording since "mmap" is used here as a verb.  It might be better
> to just leave the comment unchanged.

Sure am happy with that of course, I think Sashiko moaned about this but
it's obviously fine either way.

Andrew - do you mind restoring the comment to its original form above? Thanks!

>
> Michael
>
>
> +	int (*mmap_prepare_ring_buffer)(struct vmbus_channel *channel, struct vm_area_desc *desc);
>
>  	/* boolean to control visibility of sysfs for ring buffer */
>  	bool ring_sysfs_visible;

Cheers, Lorenzo