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

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

[Jonathan Cameron via]

Changes since RFC: (Thanks to Fan Ni)
- Fix trivial whitespace and long line issues.

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.

Based on top of:
https://lore.kernel.org/linux-cxl/20230120142450.16089-1-Jonathan.Cameron@huawei.com/
[PATCH v2 0/7] hw/cxl: RAS error emulation and injection
which is in turn on top of:
https://lore.kernel.org/all/20230112102644.27830-1-Jonathan.Cameron@huawei.com/
[PATCH v2 0/8] hw/cxl: CXL emulation cleanups and minor fixes for upstream

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 | 44 +++++++++++++++++++++++++----
 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, 101 insertions(+), 17 deletions(-)

-- 
2.37.2

[PATCH 1/2] hw/pci: Add pcie_count_ds_port() and pcie_find_port_first() helpers

[Jonathan Cameron via]

These two helpers enable host bridges to operate differently depending on
the number of downstream ports, in particular if there is only a single
port.

Useful for CXL where HDM address decoders are allowed to be implicit in
the host bridge if there is only a single root port.

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
 hw/pci/pcie_port.c         | 38 ++++++++++++++++++++++++++++++++++++++
 include/hw/pci/pcie_port.h |  2 ++
 2 files changed, 40 insertions(+)

diff --git a/hw/pci/pcie_port.c b/hw/pci/pcie_port.c
index 687e4e763a..cae22e8b28 100644
--- a/hw/pci/pcie_port.c
+++ b/hw/pci/pcie_port.c
@@ -161,6 +161,44 @@ PCIDevice *pcie_find_port_by_pn(PCIBus *bus, uint8_t pn)
     return NULL;
 }
 
+/* Find first port in devfn number order */
+PCIDevice *pcie_find_port_first(PCIBus *bus)
+{
+    int devfn;
+
+    for (devfn = 0; devfn < ARRAY_SIZE(bus->devices); devfn++) {
+        PCIDevice *d = bus->devices[devfn];
+
+        if (!d || !pci_is_express(d) || !d->exp.exp_cap) {
+            continue;
+        }
+
+        if (object_dynamic_cast(OBJECT(d), TYPE_PCIE_PORT)) {
+            return d;
+        }
+    }
+
+    return NULL;
+}
+
+int pcie_count_ds_ports(PCIBus *bus)
+{
+    int dsp_count = 0;
+    int devfn;
+
+    for (devfn = 0; devfn < ARRAY_SIZE(bus->devices); devfn++) {
+        PCIDevice *d = bus->devices[devfn];
+
+        if (!d || !pci_is_express(d) || !d->exp.exp_cap) {
+            continue;
+        }
+        if (object_dynamic_cast(OBJECT(d), TYPE_PCIE_PORT)) {
+            dsp_count++;
+        }
+    }
+    return dsp_count;
+}
+
 static const TypeInfo pcie_port_type_info = {
     .name = TYPE_PCIE_PORT,
     .parent = TYPE_PCI_BRIDGE,
diff --git a/include/hw/pci/pcie_port.h b/include/hw/pci/pcie_port.h
index fd484afb30..2cbad72555 100644
--- a/include/hw/pci/pcie_port.h
+++ b/include/hw/pci/pcie_port.h
@@ -41,6 +41,8 @@ struct PCIEPort {
 void pcie_port_init_reg(PCIDevice *d);
 
 PCIDevice *pcie_find_port_by_pn(PCIBus *bus, uint8_t pn);
+PCIDevice *pcie_find_port_first(PCIBus *bus);
+int pcie_count_ds_ports(PCIBus *bus);
 
 #define TYPE_PCIE_SLOT "pcie-slot"
 OBJECT_DECLARE_SIMPLE_TYPE(PCIESlot, PCIE_SLOT)
-- 
2.37.2

[PATCH 2/2] hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

[Jonathan Cameron via]

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>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
---
 hw/cxl/cxl-host.c                   | 31 ++++++++++++--------
 hw/pci-bridge/pci_expander_bridge.c | 44 +++++++++++++++++++++++++----
 include/hw/cxl/cxl.h                |  1 +
 include/hw/cxl/cxl_component.h      |  1 +
 include/hw/pci/pci_bridge.h         |  1 +
 5 files changed, 61 insertions(+), 17 deletions(-)

diff --git a/hw/cxl/cxl-host.c b/hw/cxl/cxl-host.c
index 3c1ec8732a..6e923ceeaf 100644
--- a/hw/cxl/cxl-host.c
+++ b/hw/cxl/cxl-host.c
@@ -146,21 +146,28 @@ static PCIDevice *cxl_cfmws_find_device(CXLFixedWindow *fw, hwaddr addr)
         return NULL;
     }
 
-    hb_cstate = cxl_get_hb_cstate(hb);
-    if (!hb_cstate) {
-        return NULL;
-    }
+    if (cxl_get_hb_passthrough(hb)) {
+        rp = pcie_find_port_first(hb->bus);
+        if (!rp) {
+            return NULL;
+        }
+    } else {
+        hb_cstate = cxl_get_hb_cstate(hb);
+        if (!hb_cstate) {
+            return NULL;
+        }
 
-    cache_mem = hb_cstate->crb.cache_mem_registers;
+        cache_mem = hb_cstate->crb.cache_mem_registers;
 
-    target_found = cxl_hdm_find_target(cache_mem, addr, &target);
-    if (!target_found) {
-        return NULL;
-    }
+        target_found = cxl_hdm_find_target(cache_mem, addr, &target);
+        if (!target_found) {
+            return NULL;
+        }
 
-    rp = pcie_find_port_by_pn(hb->bus, target);
-    if (!rp) {
-        return NULL;
+        rp = pcie_find_port_by_pn(hb->bus, target);
+        if (!rp) {
+            return NULL;
+        }
     }
 
     d = pci_bridge_get_sec_bus(PCI_BRIDGE(rp))->devices[0];
diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
index e752a21292..ead33f0c05 100644
--- a/hw/pci-bridge/pci_expander_bridge.c
+++ b/hw/pci-bridge/pci_expander_bridge.c
@@ -15,6 +15,7 @@
 #include "hw/pci/pci.h"
 #include "hw/pci/pci_bus.h"
 #include "hw/pci/pci_host.h"
+#include "hw/pci/pcie_port.h"
 #include "hw/qdev-properties.h"
 #include "hw/pci/pci_bridge.h"
 #include "hw/pci-bridge/pci_expander_bridge.h"
@@ -79,6 +80,13 @@ CXLComponentState *cxl_get_hb_cstate(PCIHostState *hb)
     return &host->cxl_cstate;
 }
 
+bool cxl_get_hb_passthrough(PCIHostState *hb)
+{
+    CXLHost *host = PXB_CXL_HOST(hb);
+
+    return host->passthrough;
+}
+
 static int pxb_bus_num(PCIBus *bus)
 {
     PXBDev *pxb = convert_to_pxb(bus->parent_dev);
@@ -289,15 +297,32 @@ static int pxb_map_irq_fn(PCIDevice *pci_dev, int pin)
     return pin - PCI_SLOT(pxb->devfn);
 }
 
-static void pxb_dev_reset(DeviceState *dev)
+static void pxb_cxl_dev_reset(DeviceState *dev)
 {
     CXLHost *cxl = PXB_CXL_DEV(dev)->cxl.cxl_host_bridge;
     CXLComponentState *cxl_cstate = &cxl->cxl_cstate;
+    PCIHostState *hb = PCI_HOST_BRIDGE(cxl);
     uint32_t *reg_state = cxl_cstate->crb.cache_mem_registers;
     uint32_t *write_msk = cxl_cstate->crb.cache_mem_regs_write_mask;
+    int dsp_count = 0;
 
     cxl_component_register_init_common(reg_state, write_msk, CXL2_ROOT_PORT);
-    ARRAY_FIELD_DP32(reg_state, CXL_HDM_DECODER_CAPABILITY, TARGET_COUNT, 8);
+    /*
+     * The CXL specification allows for host bridges with no HDM decoders
+     * if they only have a single root port.
+     */
+    if (!PXB_DEV(dev)->hdm_for_passthrough) {
+        dsp_count = pcie_count_ds_ports(hb->bus);
+    }
+    /* Initial reset will have 0 dsp so wait until > 0 */
+    if (dsp_count == 1) {
+        cxl->passthrough = true;
+        /* Set Capability ID in header to NONE */
+        ARRAY_FIELD_DP32(reg_state, CXL_HDM_CAPABILITY_HEADER, ID, 0);
+    } else {
+        ARRAY_FIELD_DP32(reg_state, CXL_HDM_DECODER_CAPABILITY, TARGET_COUNT,
+                         8);
+    }
 }
 
 static gint pxb_compare(gconstpointer a, gconstpointer b)
@@ -481,9 +506,18 @@ static void pxb_cxl_dev_realize(PCIDevice *dev, Error **errp)
     }
 
     pxb_dev_realize_common(dev, CXL, errp);
-    pxb_dev_reset(DEVICE(dev));
+    pxb_cxl_dev_reset(DEVICE(dev));
 }
 
+static Property pxb_cxl_dev_properties[] = {
+    /* Note: 0 is not a legal PXB bus number. */
+    DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
+    DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
+    DEFINE_PROP_BOOL("bypass_iommu", PXBDev, bypass_iommu, false),
+    DEFINE_PROP_BOOL("hdm_for_passthrough", PXBDev, hdm_for_passthrough, false),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
 static void pxb_cxl_dev_class_init(ObjectClass *klass, void *data)
 {
     DeviceClass *dc   = DEVICE_CLASS(klass);
@@ -497,12 +531,12 @@ static void pxb_cxl_dev_class_init(ObjectClass *klass, void *data)
      */
 
     dc->desc = "CXL Host Bridge";
-    device_class_set_props(dc, pxb_dev_properties);
+    device_class_set_props(dc, pxb_cxl_dev_properties);
     set_bit(DEVICE_CATEGORY_BRIDGE, dc->categories);
 
     /* Host bridges aren't hotpluggable. FIXME: spec reference */
     dc->hotpluggable = false;
-    dc->reset = pxb_dev_reset;
+    dc->reset = pxb_cxl_dev_reset;
 }
 
 static const TypeInfo pxb_cxl_dev_info = {
diff --git a/include/hw/cxl/cxl.h b/include/hw/cxl/cxl.h
index b161be59b7..b2cffbb364 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 8752171f70..b4104b78b5 100644
--- a/include/hw/cxl/cxl_component.h
+++ b/include/hw/cxl/cxl_component.h
@@ -249,6 +249,7 @@ 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);
diff --git a/include/hw/pci/pci_bridge.h b/include/hw/pci/pci_bridge.h
index 63a7521567..81a058bb2c 100644
--- a/include/hw/pci/pci_bridge.h
+++ b/include/hw/pci/pci_bridge.h
@@ -92,6 +92,7 @@ struct PXBDev {
     uint8_t bus_nr;
     uint16_t numa_node;
     bool bypass_iommu;
+    bool hdm_for_passthrough;
     struct cxl_dev {
         CXLHost *cxl_host_bridge; /* Pointer to a CXLHost */
     } cxl;
-- 
2.37.2

Re: [PATCH 2/2] hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

[Fan Ni]

On Wed, Jan 25, 2023 at 03:27:03PM +0000, Jonathan Cameron wrote:

> 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>
> Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> ---
>  hw/cxl/cxl-host.c                   | 31 ++++++++++++--------
>  hw/pci-bridge/pci_expander_bridge.c | 44 +++++++++++++++++++++++++----
>  include/hw/cxl/cxl.h                |  1 +
>  include/hw/cxl/cxl_component.h      |  1 +
>  include/hw/pci/pci_bridge.h         |  1 +
>  5 files changed, 61 insertions(+), 17 deletions(-)
> 
> diff --git a/hw/cxl/cxl-host.c b/hw/cxl/cxl-host.c
> index 3c1ec8732a..6e923ceeaf 100644
> --- a/hw/cxl/cxl-host.c
> +++ b/hw/cxl/cxl-host.c
> @@ -146,21 +146,28 @@ static PCIDevice *cxl_cfmws_find_device(CXLFixedWindow *fw, hwaddr addr)
>          return NULL;
>      }
>  
> -    hb_cstate = cxl_get_hb_cstate(hb);
> -    if (!hb_cstate) {
> -        return NULL;
> -    }
> +    if (cxl_get_hb_passthrough(hb)) {
> +        rp = pcie_find_port_first(hb->bus);
> +        if (!rp) {
> +            return NULL;
> +        }
> +    } else {
> +        hb_cstate = cxl_get_hb_cstate(hb);
> +        if (!hb_cstate) {
> +            return NULL;
> +        }
>  
> -    cache_mem = hb_cstate->crb.cache_mem_registers;
> +        cache_mem = hb_cstate->crb.cache_mem_registers;
>  
> -    target_found = cxl_hdm_find_target(cache_mem, addr, &target);
> -    if (!target_found) {
> -        return NULL;
> -    }
> +        target_found = cxl_hdm_find_target(cache_mem, addr, &target);
> +        if (!target_found) {
> +            return NULL;
> +        }
>  
> -    rp = pcie_find_port_by_pn(hb->bus, target);
> -    if (!rp) {
> -        return NULL;
> +        rp = pcie_find_port_by_pn(hb->bus, target);
> +        if (!rp) {
> +            return NULL;
> +        }
>      }
>  
>      d = pci_bridge_get_sec_bus(PCI_BRIDGE(rp))->devices[0];
> diff --git a/hw/pci-bridge/pci_expander_bridge.c b/hw/pci-bridge/pci_expander_bridge.c
> index e752a21292..ead33f0c05 100644
> --- a/hw/pci-bridge/pci_expander_bridge.c
> +++ b/hw/pci-bridge/pci_expander_bridge.c
> @@ -15,6 +15,7 @@
>  #include "hw/pci/pci.h"
>  #include "hw/pci/pci_bus.h"
>  #include "hw/pci/pci_host.h"
> +#include "hw/pci/pcie_port.h"
>  #include "hw/qdev-properties.h"
>  #include "hw/pci/pci_bridge.h"
>  #include "hw/pci-bridge/pci_expander_bridge.h"
> @@ -79,6 +80,13 @@ CXLComponentState *cxl_get_hb_cstate(PCIHostState *hb)
>      return &host->cxl_cstate;
>  }
>  
> +bool cxl_get_hb_passthrough(PCIHostState *hb)
> +{
> +    CXLHost *host = PXB_CXL_HOST(hb);
> +
> +    return host->passthrough;
> +}
> +
>  static int pxb_bus_num(PCIBus *bus)
>  {
>      PXBDev *pxb = convert_to_pxb(bus->parent_dev);
> @@ -289,15 +297,32 @@ static int pxb_map_irq_fn(PCIDevice *pci_dev, int pin)
>      return pin - PCI_SLOT(pxb->devfn);
>  }
>  
> -static void pxb_dev_reset(DeviceState *dev)
> +static void pxb_cxl_dev_reset(DeviceState *dev)
>  {
>      CXLHost *cxl = PXB_CXL_DEV(dev)->cxl.cxl_host_bridge;
>      CXLComponentState *cxl_cstate = &cxl->cxl_cstate;
> +    PCIHostState *hb = PCI_HOST_BRIDGE(cxl);
>      uint32_t *reg_state = cxl_cstate->crb.cache_mem_registers;
>      uint32_t *write_msk = cxl_cstate->crb.cache_mem_regs_write_mask;
> +    int dsp_count = 0;
>  
>      cxl_component_register_init_common(reg_state, write_msk, CXL2_ROOT_PORT);
> -    ARRAY_FIELD_DP32(reg_state, CXL_HDM_DECODER_CAPABILITY, TARGET_COUNT, 8);
> +    /*
> +     * The CXL specification allows for host bridges with no HDM decoders
> +     * if they only have a single root port.
> +     */
> +    if (!PXB_DEV(dev)->hdm_for_passthrough) {
> +        dsp_count = pcie_count_ds_ports(hb->bus);
> +    }
> +    /* Initial reset will have 0 dsp so wait until > 0 */
> +    if (dsp_count == 1) {
> +        cxl->passthrough = true;
> +        /* Set Capability ID in header to NONE */
> +        ARRAY_FIELD_DP32(reg_state, CXL_HDM_CAPABILITY_HEADER, ID, 0);
> +    } else {
> +        ARRAY_FIELD_DP32(reg_state, CXL_HDM_DECODER_CAPABILITY, TARGET_COUNT,
> +                         8);
> +    }
>  }
>  
>  static gint pxb_compare(gconstpointer a, gconstpointer b)
> @@ -481,9 +506,18 @@ static void pxb_cxl_dev_realize(PCIDevice *dev, Error **errp)
>      }
>  
>      pxb_dev_realize_common(dev, CXL, errp);
> -    pxb_dev_reset(DEVICE(dev));
> +    pxb_cxl_dev_reset(DEVICE(dev));
>  }
>  
> +static Property pxb_cxl_dev_properties[] = {
> +    /* Note: 0 is not a legal PXB bus number. */
> +    DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
> +    DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
> +    DEFINE_PROP_BOOL("bypass_iommu", PXBDev, bypass_iommu, false),
> +    DEFINE_PROP_BOOL("hdm_for_passthrough", PXBDev, hdm_for_passthrough, false),
when setting hdm_for_passthrough to true at the qemu command line, we
will see the segfault issue as before. I think this is expected as it
is the logic in cxl_cfmws_find_device. Wondering if there will be
following fixes to handle the case when hdm_for_passthrough is true.
> +    DEFINE_PROP_END_OF_LIST(),
> +};
> +
>  static void pxb_cxl_dev_class_init(ObjectClass *klass, void *data)
>  {
>      DeviceClass *dc   = DEVICE_CLASS(klass);
> @@ -497,12 +531,12 @@ static void pxb_cxl_dev_class_init(ObjectClass *klass, void *data)
>       */
>  
>      dc->desc = "CXL Host Bridge";
> -    device_class_set_props(dc, pxb_dev_properties);
> +    device_class_set_props(dc, pxb_cxl_dev_properties);
>      set_bit(DEVICE_CATEGORY_BRIDGE, dc->categories);
>  
>      /* Host bridges aren't hotpluggable. FIXME: spec reference */
>      dc->hotpluggable = false;
> -    dc->reset = pxb_dev_reset;
> +    dc->reset = pxb_cxl_dev_reset;
>  }
>  
>  static const TypeInfo pxb_cxl_dev_info = {
> diff --git a/include/hw/cxl/cxl.h b/include/hw/cxl/cxl.h
> index b161be59b7..b2cffbb364 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 8752171f70..b4104b78b5 100644
> --- a/include/hw/cxl/cxl_component.h
> +++ b/include/hw/cxl/cxl_component.h
> @@ -249,6 +249,7 @@ 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);
> diff --git a/include/hw/pci/pci_bridge.h b/include/hw/pci/pci_bridge.h
> index 63a7521567..81a058bb2c 100644
> --- a/include/hw/pci/pci_bridge.h
> +++ b/include/hw/pci/pci_bridge.h
> @@ -92,6 +92,7 @@ struct PXBDev {
>      uint8_t bus_nr;
>      uint16_t numa_node;
>      bool bypass_iommu;
> +    bool hdm_for_passthrough;
>      struct cxl_dev {
>          CXLHost *cxl_host_bridge; /* Pointer to a CXLHost */
>      } cxl;
> -- 
> 2.37.2
> 
> 

Re: [PATCH 2/2] hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

[Jonathan Cameron via]

On Thu, 26 Jan 2023 21:57:35 +0000
Fan Ni <fan.ni@samsung.com> wrote:

> On Wed, Jan 25, 2023 at 03:27:03PM +0000, Jonathan Cameron wrote:
> 
> > 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>
> > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

> >      pxb_dev_realize_common(dev, CXL, errp);
> > -    pxb_dev_reset(DEVICE(dev));
> > +    pxb_cxl_dev_reset(DEVICE(dev));
> >  }
> >  
> > +static Property pxb_cxl_dev_properties[] = {
> > +    /* Note: 0 is not a legal PXB bus number. */
> > +    DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
> > +    DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
> > +    DEFINE_PROP_BOOL("bypass_iommu", PXBDev, bypass_iommu, false),
> > +    DEFINE_PROP_BOOL("hdm_for_passthrough", PXBDev, hdm_for_passthrough, false),  
> when setting hdm_for_passthrough to true at the qemu command line, we
> will see the segfault issue as before. I think this is expected as it
> is the logic in cxl_cfmws_find_device. Wondering if there will be
> following fixes to handle the case when hdm_for_passthrough is true.

Absolutely, I'd expect a kernel fix for that case, but it's probably not
high priority for anyone given we don't yet have any hardware that does that
(as far as I know anyway!)

I wanted to keep the control here to make that easy to test when we do
have the fix in place.

Jonathan

Re: [PATCH 2/2] hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

[Fan Ni]

On Fri, Jan 27, 2023 at 10:01:49AM +0000, Jonathan Cameron wrote:

> On Thu, 26 Jan 2023 21:57:35 +0000
> Fan Ni <fan.ni@samsung.com> wrote:
> 
> > On Wed, Jan 25, 2023 at 03:27:03PM +0000, Jonathan Cameron wrote:
> > 
> > > 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>
> > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> 
> > >      pxb_dev_realize_common(dev, CXL, errp);
> > > -    pxb_dev_reset(DEVICE(dev));
> > > +    pxb_cxl_dev_reset(DEVICE(dev));
> > >  }
> > >  
> > > +static Property pxb_cxl_dev_properties[] = {
> > > +    /* Note: 0 is not a legal PXB bus number. */
> > > +    DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
> > > +    DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
> > > +    DEFINE_PROP_BOOL("bypass_iommu", PXBDev, bypass_iommu, false),
> > > +    DEFINE_PROP_BOOL("hdm_for_passthrough", PXBDev, hdm_for_passthrough, false),  
> > when setting hdm_for_passthrough to true at the qemu command line, we
> > will see the segfault issue as before. I think this is expected as it
> > is the logic in cxl_cfmws_find_device. Wondering if there will be
> > following fixes to handle the case when hdm_for_passthrough is true.
> 
> Absolutely, I'd expect a kernel fix for that case, but it's probably not
> high priority for anyone given we don't yet have any hardware that does that
> (as far as I know anyway!)
> 
> I wanted to keep the control here to make that easy to test when we do
> have the fix in place.
> 
> Jonathan
Make sense!

Reviwed-by: Fan Ni <fan.ni@samsung.com>
Tested-by: Fan Ni <fan.ni@samsung.com>

Re: [PATCH 2/2] hw/pxb-cxl: Support passthrough HDM Decoders unless overridden

[Jonathan Cameron via]

On Fri, 27 Jan 2023 17:02:36 +0000
Fan Ni <fan.ni@samsung.com> wrote:

> On Fri, Jan 27, 2023 at 10:01:49AM +0000, Jonathan Cameron wrote:
> 
> > On Thu, 26 Jan 2023 21:57:35 +0000
> > Fan Ni <fan.ni@samsung.com> wrote:
> >   
> > > On Wed, Jan 25, 2023 at 03:27:03PM +0000, Jonathan Cameron wrote:
> > >   
> > > > 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>
> > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>  
> >   
> > > >      pxb_dev_realize_common(dev, CXL, errp);
> > > > -    pxb_dev_reset(DEVICE(dev));
> > > > +    pxb_cxl_dev_reset(DEVICE(dev));
> > > >  }
> > > >  
> > > > +static Property pxb_cxl_dev_properties[] = {
> > > > +    /* Note: 0 is not a legal PXB bus number. */
> > > > +    DEFINE_PROP_UINT8("bus_nr", PXBDev, bus_nr, 0),
> > > > +    DEFINE_PROP_UINT16("numa_node", PXBDev, numa_node, NUMA_NODE_UNASSIGNED),
> > > > +    DEFINE_PROP_BOOL("bypass_iommu", PXBDev, bypass_iommu, false),
> > > > +    DEFINE_PROP_BOOL("hdm_for_passthrough", PXBDev, hdm_for_passthrough, false),    
> > > when setting hdm_for_passthrough to true at the qemu command line, we
> > > will see the segfault issue as before. I think this is expected as it
> > > is the logic in cxl_cfmws_find_device. Wondering if there will be
> > > following fixes to handle the case when hdm_for_passthrough is true.  
> > 
> > Absolutely, I'd expect a kernel fix for that case, but it's probably not
> > high priority for anyone given we don't yet have any hardware that does that
> > (as far as I know anyway!)
> > 
> > I wanted to keep the control here to make that easy to test when we do
> > have the fix in place.
> > Jonathan  
> Make sense!
> 
> Reviwed-by: Fan Ni <fan.ni@samsung.com>
> Tested-by: Fan Ni <fan.ni@samsung.com>

Thanks!

Typo in Reviewed-by If this version gets picked up hopefully Michael can
fix whilst applying. If I do a v2 for other feedback I'll fix it.

Jonathan