linux-kernel - RE: [PATCH v2 1/1] PCI: hv: Fix ring buffer size calculation

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Message-ID: <0a680c72-0c30-d3e7-5c67-a0aefe752be2@linux.intel.com>
Date: Mon, 19 Feb 2024 12:47:33 +0200 (EET)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Michael Kelley <mhklinux@...look.com>
cc: "haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, 
    "wei.liu@...nel.org" <wei.liu@...nel.org>, 
    "decui@...rosoft.com" <decui@...rosoft.com>, 
    "lpieralisi@...nel.org" <lpieralisi@...nel.org>, 
    "kw@...ux.com" <kw@...ux.com>, "robh@...nel.org" <robh@...nel.org>, 
    "bhelgaas@...gle.com" <bhelgaas@...gle.com>, 
    "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>, 
    LKML <linux-kernel@...r.kernel.org>, 
    "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH v2 1/1] PCI: hv: Fix ring buffer size calculation

On Fri, 16 Feb 2024, Michael Kelley wrote:

> From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com> Sent: Friday, February 16, 2024 6:46 AM
> > 
> > On Wed, 14 Feb 2024, mhkelley58@...il.com wrote:
> > >
> > > 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@...r.kernel.org> # 5.15.x
> > > Signed-off-by: Michael Kelley <mhklinux@...look.com>
> > > Reviewed-by: Kuppuswamy Sathyanarayanan
> > <sathyanarayanan.kuppuswamy@...ux.intel.com>
> > > ---
> > > Changes in v2:
> > > * Use SZ_16K instead of 16 * 1024
> > > ---
> > >  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..baadc1e5090e 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(SZ_16K);
> > >
> > >  /*
> > >   * Driver specific state.
> > >
> > 
> > Hi,
> > 
> > You forgot to add #include <linux/sizes.h> for it.
> > 
> > With that fixed:
> > 
> > Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
> > 
> 
> Fixed in v3.  I mis-interpreted your previous comment about
> adding the #include "if needed".  It's not needed to compile
> correctly, as sizes.h is indirectly included through some other
> #include.  But it's better to directly #include what's needed
> lest some unrelated change cause a failure.

Yes, we try include the headers we use in the .c file. I used "if needed" 
because I didn't check if it was already among the #includes in the file.

Our tools are lacking to enforce/check a file has correct set of #includes 
so it's currently based mostly on reviewers noticing something is wrong 
with #includes, hopefully some time in the future, the tools also catch 
up.


-- 
 i.