[PATCH v2] arm64: Enable PCI write-combine resources under sysfs

[Catalin Marinas]

On Fri, Sep 18, 2020 at 03:33:12AM +0000, Clint Sbisa wrote:
> This change exposes write-combine mappings under sysfs for
> prefetchable PCI resources on arm64.
> 
> Originally, the usage of "write combine" here was driven by the x86
> definition of write combine. This definition is specific to x86 and
> does not generalize to other architectures. However, the usage of WC
> has mutated to "write combine" semantics, which is implemented
> differently on each arch.
> 
> Generally, prefetchable BARs are accepted to allow speculative
> accesses, write combining, and re-ordering-- from the PCI perspective,
> this means there are no read side effects. (This contradicts the PCI
> spec which allows prefetchable BARs to have read side effects, but
> this definition is ill-advised as it is impossible to meet.) On x86,
> prefetchable BARs are mapped as WC as originally defined (with some
> conditionals on arch features). On arm64, WC is taken to mean normal
> non-cacheable memory.
> 
> In practice, write combine semantics are used to minimize write
> operations. A common usage of this is minimizing PCI TLPs which can
> significantly improve performance with PCI devices. In order to
> provide the same benefits to userspace, we need to allow userspace to
> map prefetchable BARs with write combine semantics. The resourceX_wc
> mapping is used today by userspace programs and libraries.
> 
> While this model is flawed as "write combine" is very ill-defined, it
> is already used by multiple non-x86 archs to expose write combine
> semantics to user space. We enable this on arm64 to give userspace on
> arm64 an equivalent mechanism for utilizing write combining with PCI
> devices.
> 
> Cc: Benjamin Herrenschmidt <benh at kernel.crashing.org>
> Cc: Bjorn Helgaas <helgaas at kernel.org>
> Cc: Catalin Marinas <catalin.marinas at arm.com>
> Cc: Jason Gunthorpe <jgg at nvidia.com>
> Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> Cc: Will Deacon <will at kernel.org>
> Signed-off-by: Clint Sbisa <csbisa at amazon.com>

Acked-by: Catalin Marinas <catalin.marinas at arm.com>