linux-kernel - RE: [PATCH v3 1/1] PCI: hv: Fix ring buffer size calculation

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Date: Fri, 16 Feb 2024 23:40:53 +0000
From: Long Li <longli@...rosoft.com>
To: "mhklinux@...look.com" <mhklinux@...look.com>, Haiyang Zhang
	<haiyangz@...rosoft.com>, "wei.liu@...nel.org" <wei.liu@...nel.org>, Dexuan
 Cui <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>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>
Subject: RE: [PATCH v3 1/1] PCI: hv: Fix ring buffer size calculation

> 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>
> Reviewed-by: Kuppuswamy Sathyanarayanan
> <sathyanarayanan.kuppuswamy@...ux.intel.com>
> Reviewed-by: Ilpo Jarvinen <ilpo.jarvinen@...ux.intel.com>
> 
> ---
> Changes in v3:
> * Add #include of sizes.h
> Changes in v2:
> * Use SZ_16K instead of 16 * 1024
> ---
>  drivers/pci/controller/pci-hyperv.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-
> hyperv.c
> index 1eaffff40b8d..5992280e8110 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -49,6 +49,7 @@
>  #include <linux/refcount.h>
>  #include <linux/irqdomain.h>
>  #include <linux/acpi.h>
> +#include <linux/sizes.h>
>  #include <asm/mshyperv.h>
> 
>  /*
> @@ -465,7 +466,7 @@ 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.
> --
> 2.25.1
> 

Reviewed-by: Long Li <longli@...rosoft.com>