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

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Date: Mon, 11 Mar 2024 03:21:32 +0900
From: Krzysztof Wilczyński <kw@...ux.com>
To: mhklinux@...look.com
Cc: haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
	lpieralisi@...nel.org, robh@...nel.org, bhelgaas@...gle.com,
	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-hyperv@...r.kernel.org
Subject: Re: [PATCH v3 1/1] PCI: hv: Fix ring buffer size calculation

Hello,

> 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.

Applied to controller/hyperv, thank you!

[1/1] PCI: hv: Fix ring buffer size calculation
      https://git.kernel.org/pci/pci/c/192c0b72019f

	Krzysztof