Re: [RFC PATCH 0/2] hw/cxl: Passthrough HDM decoder emulation

[Fan Ni]

On Tue, Jan 24, 2023 at 09:47:20AM +0000, Jonathan Cameron wrote:

> On Mon, 23 Jan 2023 17:53:24 +0000
> Fan Ni <fan.ni@samsung.com> wrote:
> 
> > On Mon, Jan 23, 2023 at 12:17:10PM +0000, Jonathan Cameron wrote:
> > 
> > 
> > 
> > > Until now, testing using CXL has relied up always using two root ports
> > > below a host bridge, to work around a current assumption in the Linux
> > > kernel support that, in the single root port case, the implementation will
> > > use the allowed passthrough decoder implementation choice. If that choice
> > > is made all accesses are routed from the host bridge to the single
> > > root port that is present. Effectively we have a pass through decoder
> > > (it is called that in the kernel driver).
> > > 
> > > This patch series implements that functionality and makes it the default
> > > See patch 2 for a discussion of why I think we can make this change
> > > without backwards compatibility issues (basically if it didn't work before
> > > who are we breaking by making it work?)
> > > 
> > > Whilst this limitation has been known since the initial QEMU patch
> > > postings / kernel CXL region support, Fan Ni Ran into it recently 
> > > reminding
> > > me that we should solve it.
> > > 
> > > https://urldefense.com/v3/__https://lore.kernel.org/linux-cxl/20230113171044.GA24788@bgt-140510-bm03/__;!!EwVzqGoTKBqv-0DWAJBm!WsD6FV-KV_YnhhHWLll72cHLqLQ_8kpps3MlpAa6Bonsdz6aifuWT40-QnlRyFqWyeyaVb-RiC03_qajbeCGsI5DcPYv$
> > >  
> > > 
> > > Tree with a large set of patches before this at:
> > > https://urldefense.com/v3/__https://gitlab.com/jic23/qemu/-/tree/cxl-2023-01-20__;!!EwVzqGoTKBqv-0DWAJBm!WsD6FV-KV_YnhhHWLll72cHLqLQ_8kpps3MlpAa6Bonsdz6aifuWT40-QnlRyFqWyeyaVb-RiC03_qajbeCGsPjbv12T$
> > >  
> > > 
> > > I've done some basic testing, though I did hit what appears to be
> > > a kernel race on region bring up of existing region / namespace in a
> > > 1HB 2RP 2EP test case. That is proving hard to replicate consistently
> > > but doesn't seem to have anything to do with the emulation other than
> > > perhaps we are opening up a race by responding slowly to something.
> > > 
> > > Jonathan Cameron (2):
> > >   hw/pci: Add pcie_count_ds_port() and pcie_find_port_first() helpers
> > >   hw/pxb-cxl: Support passthrough HDM Decoders unless overridden
> > > 
> > >  hw/cxl/cxl-host.c                   | 31 +++++++++++++--------
> > >  hw/pci-bridge/pci_expander_bridge.c | 43 +++++++++++++++++++++++++----
> > >  hw/pci/pcie_port.c                  | 38 +++++++++++++++++++++++++
> > >  include/hw/cxl/cxl.h                |  1 +
> > >  include/hw/cxl/cxl_component.h      |  1 +
> > >  include/hw/pci/pci_bridge.h         |  1 +
> > >  include/hw/pci/pcie_port.h          |  2 ++
> > >  7 files changed, 100 insertions(+), 17 deletions(-)
> > > 
> > > -- 
> > > 2.37.2
> > > 
> > >   
> > Tried three different cxl topology setups (1HB1RP, 1HB2RP2Memdev, with
> > switch), the patch works fine for me.
> > Btw, there seem some format issues with the patch, got warnings with
> > checkpatch tool.
> Thanks! I'll clean those up.  Was being lazy on it as it's an RFC for
> now :)  Given this is small and useful I'll probably pull it nearer the
> head of the queue.
> 
> When I repost, if you could give a Tested-by tag that would be great!
> 
> Thanks,
> 
> Jonathan
> 
Will do. Thanks.