Re: [PATCH v7 1/6] riscv: mm: dma-noncoherent: Switch using function pointers for cache management

[Conor Dooley]

On Tue, Apr 04, 2023 at 08:50:16AM +0200, Arnd Bergmann wrote:
> On Tue, Apr 4, 2023, at 07:29, Christoph Hellwig wrote:
> > On Thu, Mar 30, 2023 at 09:42:12PM +0100, Prabhakar wrote:
> >> From: Lad Prabhakar <prabhakar.mahadev-lad.rj@xxxxxxxxxxxxxx>
> >> 
> >> Currently, selecting which CMOs to use on a given platform is done using
> >> and ALTERNATIVE_X() macro. This was manageable when there were just two
> >> CMO implementations, but now that there are more and more platforms coming
> >> needing custom CMOs, the use of the ALTERNATIVE_X() macro is unmanageable.
> >> 
> >> To avoid such issues this patch switches to use of function pointers
> >> instead of ALTERNATIVE_X() macro for cache management (the only drawback
> >> being performance over the previous approach).
> >> 
> >> void (*clean_range)(unsigned long addr, unsigned long size);
> >> void (*inv_range)(unsigned long addr, unsigned long size);
> >> void (*flush_range)(unsigned long addr, unsigned long size);
> >> 
> >
> > NAK.  Function pointers for somthing high performance as cache
> > maintainance is a complete no-go.
> 
> As we already discussed, this is now planned to use a direct
> branch to the zicbom version when the function pointer is NULL,
> which should be a fast predicted branch on all standard implementations
> and only be slow on the nonstandard ones, which I think is a reasonable
> compromise.
> 
> I'm also not sure I'd actually consider this a fast path, since
> there is already a significant cost in accessing the invalidated
> cache lines afterwards, which is likely going to be much higher than
> the cost of an indirect branch.
> 
> I suppose an alternative would be to use the linux/static_call.h
> infrastructure to avoid the overhead of indirect branches altogether.
> Right now, this has no riscv specific implementation though, so
> using it just turns into a regular indirect branch until someone
> implements static_call.

Someone actually posted an implementation for that the other day:
https://patchwork.kernel.org/project/linux-riscv/patch/tencent_A8A256967B654625AEE1DB222514B0613B07@xxxxxx/

I haven't looked at it at all, other than noticing the build issues
outside of for !rv64 || !mmu, so I have no idea what state it is
actually in.

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