[PATCH 1/5] block: new sector copy api

[Christoph Hellwig]

On Fri, May 23, 2025 at 11:02:16AM -0600, Keith Busch wrote:
> > Maybe having common code is good to avoid copies, but I suspect most
> > real users would want their own.
> 
> But the end goal is that no host memory buffer would be needed at all.
> A buffer is allocated only in the fallback path. If we have the caller
> provide their buffer for that fallback case, that kind of defeats the
> benefit of reduced memory utilization. So it sounds like you might be
> suggesting that I don't even bother providing the host instrumented copy
> fallback and provide an API that performs the copy only if it can be
> offloaded. Yes?

Well, I'd expect most uses cases of things like GC to use the "fallback"
for now - because copy isn't actually that often supported, and when it
is performance might not be good enough.  So we'll need to optimize
for both cases.  Maybe a fallback code is useful, but I'd leave the
buffer management for it to the caller so that it can reuse them.  Or
maybe don't bother given how trivial the code is anyway.  The use case
for common code would be strong if it refactored existing code and
showed that existing callers actually have enough common logic that
it's worth the effort.