Re: [PATCH v3 4/7] hw/arm/smmuv3-accel: Implement "auto" value for "ssidsize"

[Nathan Chen]

On 5/14/2026 7:24 AM, Shameer Kolothum Thodi wrote:

-----Original Message-----
From: Nathan Chen<nath...@nvidia.com>
Sent: 12 May 2026 20:35
To:qemu-...@nongnu.org;qemu-devel@nongnu.org
Cc: Eric Auger<eric.au...@redhat.com>; Peter Maydell
<peter.mayd...@linaro.org>; Philippe Mathieu-Daudé<phi...@linaro.org>;
Zhao Liu<zhao1....@intel.com>; Matt Ochs<mo...@nvidia.com>; Nicolin
Chen<nicol...@nvidia.com>; Shameer Kolothum Thodi
<skolothum...@nvidia.com>; Nathan Chen<nath...@nvidia.com>
Subject: [PATCH v3 4/7] hw/arm/smmuv3-accel: Implement "auto" value for
"ssidsize"

From: Nathan Chen<nath...@nvidia.com>

Allow accelerated SMMUv3 SSID size property to be derived from host
IOMMU capabilities. Derive host values using IOMMU_GET_HW_INFO,
retrieving SSID size from IDR1. When the auto SSID size is resolved
to a non-zero value, PASID capability is advertised to the vIOMMU
and accelerated use cases such as Shared Virtual Addressing (SVA)
are supported.

Reviewed-by: Eric Auger<eric.au...@redhat.com>
Signed-off-by: Nathan Chen<nath...@nvidia.com>
---
  hw/arm/smmuv3-accel.c | 18 ++++++++++++++++--
  hw/arm/smmuv3.c       | 20 ++++++++++----------
  2 files changed, 26 insertions(+), 12 deletions(-)

diff --git a/hw/arm/smmuv3-accel.c b/hw/arm/smmuv3-accel.c
index a5973bd499..5b1af062ee 100644
--- a/hw/arm/smmuv3-accel.c
+++ b/hw/arm/smmuv3-accel.c
@@ -62,6 +62,12 @@ static void
smmuv3_accel_auto_finalise(SMMUv3State *s,
                                 FIELD_EX32(info->idr[3], IDR3, RIL));
      }

+    if (s->ssidsize == SSID_SIZE_MODE_AUTO) {
+        /* Store for get_viommu_flags() to determine PASID support */
+        s->idr[1] = FIELD_DP32(s->idr[1], IDR1, SSIDSIZE,
+                               FIELD_EX32(info->idr[1], IDR1, SSIDSIZE));
Nit: Not sure that comment makes sense. We need to update it
anyway, right?
After ssidsize is stored in s->idr[1], smmuv3_accel_get_viommu_flags() 
will call smmuv3_pasid_supported() and enable the 
VIOMMU_FLAG_PASID_SUPPORTED flag if AUTO is specified for ssidsize and 
s->idr[1] is non-zero. Maybe we can remove the comment or come up with a 
better wording such as:

/* PASID support will be enabled if ssidsize is non-zero */

But this is already documented in qemu-options.hx. I think it would be 
cleaner to omit the comment here. What do you think?

Thanks,
Nathan


+    }
+
      accel->auto_finalised = true;
  }

@@ -822,6 +828,13 @@ static AddressSpace
*smmuv3_accel_find_add_as(PCIBus *bus, void *opaque,
      }
  }

+static inline bool smmuv3_pasid_supported(SMMUv3State *s)
+{
+    return s->ssidsize > SSID_SIZE_MODE_0 ||
+           (s->ssidsize == SSID_SIZE_MODE_AUTO &&
+            FIELD_EX32(s->idr[1], IDR1, SSIDSIZE));
+}
+
  static uint64_t smmuv3_accel_get_viommu_flags(void *opaque)
  {
      /*
@@ -834,7 +847,7 @@ static uint64_t
smmuv3_accel_get_viommu_flags(void *opaque)
      SMMUState *bs = opaque;
      SMMUv3State *s = ARM_SMMUV3(bs);

-    if (s->ssidsize > SSID_SIZE_MODE_0) {
+    if (smmuv3_pasid_supported(s)) {
          flags |= VIOMMU_FLAG_PASID_SUPPORTED;
      }
      return flags;
@@ -975,7 +988,8 @@ bool smmuv3_accel_init(SMMUv3State *s, Error
**errp)
      smmuv3_accel_as_init(s);

      if (s->ats == ON_OFF_AUTO_AUTO ||
-        s->ril == ON_OFF_AUTO_AUTO) {
+        s->ril == ON_OFF_AUTO_AUTO ||
+        s->ssidsize == SSID_SIZE_MODE_AUTO) {
          s->s_accel->auto_mode = true;
      }

diff --git a/hw/arm/smmuv3.c b/hw/arm/smmuv3.c
index 7ec4241ecf..cd30275717 100644
--- a/hw/arm/smmuv3.c
+++ b/hw/arm/smmuv3.c
@@ -626,7 +626,10 @@ static int decode_ste(SMMUv3State *s,
SMMUTransCfg *cfg,
      }

      /* Multiple context descriptors require SubstreamID support */
-    if (s->ssidsize == SSID_SIZE_MODE_0 && STE_S1CDMAX(ste) != 0) {
+    if ((s->ssidsize == SSID_SIZE_MODE_0 ||
+         (s->ssidsize == SSID_SIZE_MODE_AUTO &&
+          !FIELD_EX32(s->idr[1], IDR1, SSIDSIZE))) &&
+        STE_S1CDMAX(ste) != 0) {
          qemu_log_mask(LOG_UNIMP,
                  "SMMUv3: multiple S1 context descriptors require SubstreamID
support. "
                  "Configure ssidsize > 0 (requires accel=on)\n");
@@ -1965,10 +1968,6 @@ static void smmu_reset_exit(Object *obj,
ResetType type)

  static bool smmu_validate_property(SMMUv3State *s, Error **errp)
  {
-    if (s->ssidsize == SSID_SIZE_MODE_AUTO) {
-        error_setg(errp, "ssidsize auto mode is not supported");
-        return false;
-    }
      if (s->oas != OAS_MODE_44 && s->oas != OAS_MODE_48) {
          error_setg(errp, "QEMU SMMUv3 model only implements 44 and 48 bit"
                     "OAS; other OasMode values are not supported");
@@ -1989,7 +1988,8 @@ static bool
smmu_validate_property(SMMUv3State *s, Error **errp)
              return false;
          }
          if (s->ssidsize > SSID_SIZE_MODE_0) {
-            error_setg(errp, "ssidsize can only be set if accel=on");
+            error_setg(errp, "ssidsize can only be greater than 0 "
+                       "bits if accel=on");
              return false;
          }
          return true;
@@ -2173,11 +2173,11 @@ static void smmuv3_class_init(ObjectClass
*klass, const void *data)
          "are 44 or 48 bits. Defaults to 44 bits. oas=auto is not "
          "supported.");
      object_class_property_set_description(klass, "ssidsize",
-        "Number of bits used to represent SubstreamIDs (SSIDs). "
+        "Set number of bits used to represent SubstreamIDs (SSIDs). "
+        "Valid values are 0-20 and auto. Defaults to 0. "
          "A value of N allows SSIDs in the range [0 .. 2^N - 1]. "
-        "Valid range is 0-20, where 0 disables SubstreamID support. "
-        "Defaults to 0. A value greater than 0 is required to enable "
-        "PASID support. ssidsize=auto is not supported.");
+        "A value of 0 disables SubstreamID support. A value greater "
+        "than 0 is required to enable PASID support.");
  }

  static int smmuv3_notify_flag_changed(IOMMUMemoryRegion *iommu,