RE: [PATCH net 1/1] hv_netvsc: Calculate correct ring size when PAGE_SIZE is not 4 Kbytes

Michael Kelley
2024-01-23T17:13:14Z

From: Simon Horman @ 2024-01-22 20:49 UTC (permalink / raw)
>
> On Mon, Jan 22, 2024 at 08:20:28AM -0800, mhkelley58@gmail.com wrote:
> > From: Michael Kelley <mhklinux@outlook.com>
> >
> > Current code in netvsc_drv_init() incorrectly assumes that PAGE_SIZE
> > is 4 Kbytes, which is wrong on ARM64 with 16K or 64K page size. As a
> > result, the default VMBus ring buffer size on ARM64 with 64K page size
> > is 8 Mbytes instead of the expected 512 Kbytes. While this doesn't break
> > anything, a typical VM with 8 vCPUs and 8 netvsc channels wastes 120
> > Mbytes (8 channels * 2 ring buffers/channel * 7.5 Mbytes/ring buffer).
> >
> > Unfortunately, the module parameter specifying the ring buffer size
> > is in units of 4 Kbyte pages. Ideally, it should be in units that
> > are independent of PAGE_SIZE, but backwards compatibility prevents
> > changing that now.
> >
> > Fix this by having netvsc_drv_init() hardcode 4096 instead of using
> > PAGE_SIZE when calculating the ring buffer size in bytes. Also
> > use the VMBUS_RING_SIZE macro to ensure proper alignment when running
> > with page size larger than 4K.
> >
> > Cc: <stable@vger.kernel.org> # 5.15.x
> > Signed-off-by: Michael Kelley <mhklinux@outlook.com>
>
> Hi Michael,
>
> As a bug fix this probably warrants a fixes tag.
> Perhaps this is appropriate?
>
> Fixes: 450d7a4b7ace ("Staging: hv: ring parameter")
>

[This email is cobbled together because for some reason I didn't directly
receive your original reply. So it won't thread correctly with yours.]

I thought about a Fixes: tag, but the situation is a bit weird. The original
code was correct enough at the time it was written in 2010 because Hyper-V
only ran on x86/x64 with a 4 Kbyte guest page size. In fact, all the Hyper-V
guest code in the Linux kernel tended to assume a 4 Kbyte page size.
During 2019 and 2020, I and others made changes to remove this
assumption, in prep for running Hyper-V Linux guests on ARM64. The
ARM64 support was finally enabled with commit 7aff79e297ee in August
2021 for the 5.15 kernel. Somehow we missed fixing this case in the netvsc
driver, and a similar case in the Hyper-V synthetic storage driver (see [1]).

As a result, there's no point in backporting this fix to anything earlier than
5.15, because there's no ARM64 support for Hyper-V guests in earlier kernels.
So picking a "Fixes:" commit from back in 2010 doesn't seem helpful. I could
see doing

Fixes: 7aff79e297ee ("Drivers: hv: Enable Hyper-V code to be built on ARM64")

But the connection between that commit and this fix isn't very evident, so I
opt'ed for just putting the 5.15.x notation on the Cc: stable@vger.kernel.org
line. That said, I don't feel strongly about it. I'm just trying to do what's best
for the stable branch maintainers and avoid generating backports to kernel
versions where it doesn't matter.

Michael

[1] https://lore.kernel.org/linux-hyperv/20240122170956.496436-1-mhklinux@outlook.com/T/#u