RE: [PATCH 1/1] PCI: hv: Fix ring buffer size calculation

[Michael Kelley]

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> 
> On 2/12/24 10:19 PM, mhkelley58@xxxxxxxxx wrote:
> > From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> >
> > For a physical PCI device that is passed through to a Hyper-V guest VM,
> > current code specifies the VMBus ring buffer size as 4 pages.  But this
> > is an inappropriate dependency, since the amount of ring buffer space
> > needed is unrelated to PAGE_SIZE. For example, on x86 the ring buffer
> > size ends up as 16 Kbytes, while on ARM64 with 64 Kbyte pages, the ring
> > size bloats to 256 Kbytes. The ring buffer for PCI pass-thru devices
> > is used for only a few messages during device setup and removal, so any
> > space above a few Kbytes is wasted.
> >
> > Fix this by declaring the ring buffer size to be a fixed 16 Kbytes.
> > Furthermore, use the VMBUS_RING_SIZE() macro so that the ring buffer
> > header is properly accounted for, and so the size is rounded up to a
> > page boundary, using the page size for which the kernel is built. While
> > w/64 Kbyte pages this results in a 64 Kbyte ring buffer header plus a
> > 64 Kbyte ring buffer, that's the smallest possible with that page size.
> > It's still 128 Kbytes better than the current code.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx> # 5.15.x
> > Signed-off-by: Michael Kelley <mhklinux@xxxxxxxxxxx>
> > ---
> Looks good to me.
> 
> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> >  drivers/pci/controller/pci-hyperv.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> > index 1eaffff40b8d..5f22ad38bb98 100644
> > --- a/drivers/pci/controller/pci-hyperv.c
> > +++ b/drivers/pci/controller/pci-hyperv.c
> > @@ -465,7 +465,7 @@ struct pci_eject_response {
> >  	u32 status;
> >  } __packed;
> >
> > -static int pci_ring_size = (4 * PAGE_SIZE);
> > +static int pci_ring_size = VMBUS_RING_SIZE(16 * 1024);
> >
> Nit: I think you can use SZ_16K to make it more readable.

Thanks for the review.  I'll send a v2 that uses SZ_16K per your
and Ilpo Järvinen's comment.

Michael