Re: [PATCH 0/2] i440fx chipset and larger system memory totals

Hello,

Michael Kelly, le mer. 26 août 2026 20:53:54 +0100, a ecrit:
> On 22/08/2026 08:38, Michael Kelly wrote:
> > I'm considering whether it would be appropriate to implement within rump
> > a similar bounce buffer capability as is found in x86. I've only looked
> > briefly at this code and in general it is arch independent except for
> > clearly arch specific functionality relating to the the memory sync
> > requirements of DMA and user buffers. There is however arch specific
> > code for rumpkern so possibly having arch specific variants of 
> > bus_dmamap_sync within rump might be feasible.
> 
> I've prototyped this successfully such that Hurd boots using Qemu i440fx
> machine type and 6G RAM without the patch that has currently been deployed.
> This has several advantages:
> 
> 1) Using q35 machine reverts to the previous performance without using
> bounce buffers at all.
> 
> 2) Using i440fx uses a bounce buffer only where the physical memory address
> requires it. The bounce buffer is retained by the IDE channel DMA map
> meaning it does not need reallocation per request.
> 
> All I've done, really, is to duplicate the relevant parts from the x86 DMA
> implementation which is mostly architecture independent.
> 
> If this is deemed the best solution, what is the way forward for integration
> ? I'm assuming that it would be preferred to see if I can get this
> functionality merged to the upstream rumpkernel sources but I think that
> might take quite a long time. They might not be happy with the idea let
> alone the implementation. There's quite a bit of work to transform the
> prototype into something that they might consider. For example, there are
> functions that I've not revised (bus_dmamap_load_mbuf and
> bus_dmamap_load_uio) that Hurd does not use but NetBSD might.
> 
> Would you want an interim patch to replace the existing one or just leave
> things alone for now ?

If you have a working patch we can use it to get the performance
benefit while discussion with upstream happens, yes.

Thanks,
Samuel