hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

The CXL r3.0 specification allows for there to be no HDM decoders on CXL
Host Bridges if they have only a single root port. Instead, all accesses
directed to the host bridge (as specified in CXL Fixed Memory Windows)
are assumed to be routed to the single root port.

Linux currently assumes this implementation choice. So to simplify testing,
make QEMU emulation also default to no HDM decoders under these particular
circumstances, but provide a hdm_for_passthrough boolean option to have
HDM decoders as previously.

Technically this is breaking backwards compatibility, but given the only
known software stack used with the QEMU emulation is the Linux kernel
and this configuration did not work before this change, there are
unlikely to be any complaints that it now works. The option is retained
to allow testing of software that does allow for these HDM decoders to exist,
once someone writes it.

Reported-by: Fan Ni <fan.ni@samsung.com>
Reviewed-by: Fan Ni <fan.ni@samsung.com>
Tested-by: Fan Ni <fan.ni@samsung.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

--
v2: Pick up and fix typo in tag from Fan Ni
Message-Id: <20230227153128.8164-3-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

2 files changed, 2 insertions, 0 deletions

diff --git a/include/hw/cxl/cxl.h b/include/hw/cxl/cxl.h
index b161be5..b2cffbb 100644
--- a/include/hw/cxl/cxl.h
+++ b/include/hw/cxl/cxl.h
@@ -49,6 +49,7 @@ struct CXLHost {
     PCIHostState parent_obj;
 
     CXLComponentState cxl_cstate;
+    bool passthrough;
 };
 
 #define TYPE_PXB_CXL_HOST "pxb-cxl-host"
diff --git a/include/hw/cxl/cxl_component.h b/include/hw/cxl/cxl_component.h
index ec4203b..42c7e58 100644
--- a/include/hw/cxl/cxl_component.h
+++ b/include/hw/cxl/cxl_component.h
@@ -247,6 +247,7 @@ static inline hwaddr cxl_decode_ig(int ig)
 }
 
 CXLComponentState *cxl_get_hb_cstate(PCIHostState *hb);
+bool cxl_get_hb_passthrough(PCIHostState *hb);
 
 void cxl_doe_cdat_init(CXLComponentState *cxl_cstate, Error **errp);
 void cxl_doe_cdat_release(CXLComponentState *cxl_cstate);