Re: [PATCH v2 1/4] softmmu: Support concurrent bounce buffers

[Mattias Nissler]

On Tue, Sep 5, 2023 at 3:45 PM Peter Xu <peterx@redhat.com> wrote:
>
> On Tue, Sep 05, 2023 at 09:38:39AM +0200, Mattias Nissler wrote:
> > It would be nice to use a property on the device that originates the
> > DMA operation to configure this. However, I don't see how to do this
> > in a reasonable way without bigger changes: A typical call path is
> > pci_dma_map -> dma_memory_map -> address_space_map. While pci_dma_map
> > has a PCIDevice*, address_space_map only receives the AddressSpace*.
> > So, we'd probably have to pass through a new QObject parameter to
> > address_space_map that indicates the originator and pass that through?
> > Or is there a better alternative to supply context information to
> > address_space map? Let me know if any of these approaches sound
> > appropriate and I'll be happy to explore them further.
>
> Should be possible to do. The pci address space is not shared but
> per-device by default (even if there is no vIOMMU intervention).  See
> do_pci_register_device():
>
>     address_space_init(&pci_dev->bus_master_as,
>                        &pci_dev->bus_master_container_region, pci_dev->name);

Ah, thanks for that hint! This works, and it probably even makes more
sense to treat bounce buffering as a concept tied to AddressSpace
rather than a global thing.

I'll send an updated series shortly, with the configuration parameter
attached to the PCI device, so it can be specified as a -device option
on the command line. In that light, I decided to keep the default at
4096 bytes though, since we now have the ability for each device model
to choose its default independently.