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

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Date: Wed, 14 Feb 2024 16:18:38 +0200 (EET)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: mhklinux@...look.com
cc: haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com, 
    lpieralisi@...nel.org, kw@...ux.com, robh@...nel.org, bhelgaas@...gle.com, 
    linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
    linux-hyperv@...r.kernel.org
Subject: Re: [PATCH 1/1] PCI: hv: Fix ring buffer size calculation

On Mon, 12 Feb 2024, mhkelley58@...il.com wrote:

> From: Michael Kelley <mhklinux@...look.com>
> 
> 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>
> ---
>  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);

SZ_16K (and add the relevant #include if needed).

-- 
 i.