Re: [RFC] add __iomem cookie for EPF BAR

[mani <mani@kernel.org>]

On Mon, Aug 07, 2023 at 08:28:30PM +0800, Li Chen wrote:
> Hi All
> 
> Currently, the EPF's bar is allocated by pci_epf_alloc_space, which internally uses dma_alloc_coherent and the caching behavior of dma_alloc_coherent may vary depending on platforms.
> 
> The bar space is exported to RC, which means that RC may modify it without EP being aware of it, so EP still read from the cache and get stalled data. To address this issue, the bar space should be treated as iomem instead and forced to use io read/write APIs, which enforces volatile. 
> 

We already had a similar discussion on using volatile for BAR space and settled
with {WRITE/READ}_ONCE macros in EPF Test driver [1].

Since the BAR space allocated in endpoint is not a MMIO, I don't think it should
be forced as iomem. Rather EPF drivers should use _ONCE macros to access the
fields to avoid coherency issues as suggested by Arnd earlier.

- Mani

[1] https://lore.kernel.org/linux-pci/c49df2f9-9848-45aa-9d64-9e4e9841440f@app.fastmail.com/

> If you agree, I would create patches for existing EPF and EPF/EPC core and submit them for review.
> 
> Regards,
> Li

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