RE: [PATCH v2 0/8] PCI: Align small (<4k) BARs

[David Laight <David.Laight@ACULAB.COM>]

From: Stewart Hildebrand
> Sent: 16 July 2024 20:33
> 
> This series sets the default minimum resource alignment to 4k for memory
> BARs. In preparation, it makes an optimization and addresses some corner
> cases observed when reallocating BARs. I consider the prepapatory
> patches to be prerequisites to changing the default BAR alignment.

Should the BARs be page aligned on systems with large pages?
At least as an option for hypervisor pass-through and any than can be mmap()ed
into userspace.

Does any hardware actually have 'silly numbers' of small memory BARs?

I have a vague memory of some ethernet controllers having lots of (?)
virtual devices that might have separate registers than can be mapped
out to a hypervisor.
Expanding those to a large page might be problematic - but needed for security.

For more normal hardware just ensuring that two separate targets don't share
a page while allowing (eg) two 1k BAR to reside in the same 64k page would
give some security.

Aligning a small MSIX BAR is unlikely to have any effect on the address
space utilisation (for PCIe) since the bridge will assign a power of two
sized block - with a big pad (useful for generating pcie errors!)

	David

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