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

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Date: Mon, 12 Feb 2024 22:19:10 -0800
From: mhkelley58@...il.com
To: 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,
	linux-kernel@...r.kernel.org,
	linux-hyperv@...r.kernel.org
Subject: [PATCH 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>
---
 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);

 /*
  * Driver specific state.
-- 
2.25.1