RE: [PATCH 2/2] drm/hyperv: use VMBUS_RING_SIZE()

[Dexuan Cui]

> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Sent: Monday, April 27, 2026 12:06 PM
> > IMO the Fixes tag is unnecessary because the existing
> > VMBUS_RING_BUFSIZE
> > is 256KB, which is already aligned to 4KB, 16KB and 64KB.
> >
> > VMBUS_RING_SIZE(256 * 1024) is still 256KB.
> 
> Not always. If PAGE_SIZE is 64KiB, VMBUS_RING_SIZE(256 * 1024) is
> 320KiB. If PAGE_SIZE is 16KiB or 4KiB, then VMBUS_RING_SIZE(256 * 1024)
> is indeed 256 KiB. See the explanation in the comment for
> VMBUS_RING_SIZE.
> 
> Michael

Thanks for correcting me!

I didn't realize that sizeof(struct hv_ring_buffer) is based on
PAGE_SIZE, not on HV_HYP_PAGE_SIZE.

However,  it looks like the Fixes tag is still not needed:

without the patch, we always pass two arguments of 256KB to
vmbus_open().

with the patch, we still pass 256KB to vmbus_open() in the case of
PAGE_SIZE=4KB or 16KB, and we pass 320KB in the case of
PAGE_SIZE=64KB.

Both 320K and 256KB are multiples of PAGE_SIZE, so
vmbus_open() -> vmbus_alloc_ring() doesn't return -EINVAL.

In the case of PAGE_SIZE=64KB, it's OK to pass 256KB to vmbus_open()
here since the hyperv-drm driver doesn't really have to use a slightly
bigger VMBus ringbuffer size.

Thanks,
Dexuan