Re: [PATCH] hw/riscv: riscv-iommu: Don't look up DDT cache in Off and Bare modes

[Daniel Henrique Barboza]

On 10/28/25 5:50 AM, frank.ch...@sifive.com wrote:

From: Frank Chang <frank.ch...@sifive.com>

According to the RISC-V IOMMU specification:

* When ddtp.iommu_mode is set to Off, there is no DDT look-up, and an "All
   inbound transactions disallowed" fault (cause = 256) is reported for any
   inbound transaction.

* When ddtp.iommu_mode is set to Bare, there is no DDT look-up, and the
   translated address is the same as the IOVA, unless the transaction type
   is disallowed (cause = 260).

In the current implementation, the DDT cache is incorrectly looked up
even when ddtp.iommu_mode is set to Off or Bare. This may result in
unintended cache hits.

Therefore, the DDT cache must not be looked up when ddtp.iommu_mode is
set to Off or Bare. For other modes, software is required to issue cache
invalidation commands before any inbound transactions.

Signed-off-by: Frank Chang <frank.ch...@sifive.com>
---

Reviewed-by: Daniel Henrique Barboza <dbarb...@ventanamicro.com>

  hw/riscv/riscv-iommu.c | 13 +++++++++----
  1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/hw/riscv/riscv-iommu.c b/hw/riscv/riscv-iommu.c
index b33c7fe3259..95db811f125 100644
--- a/hw/riscv/riscv-iommu.c
+++ b/hw/riscv/riscv-iommu.c
@@ -1290,13 +1290,18 @@ static RISCVIOMMUContext 
*riscv_iommu_ctx(RISCVIOMMUState *s,
          .devid = devid,
          .process_id = process_id,
      };
+    unsigned mode = get_field(s->ddtp, RISCV_IOMMU_DDTP_MODE);

ctx_cache = g_hash_table_ref(s->ctx_cache);

-    ctx = g_hash_table_lookup(ctx_cache, &key);

- if (ctx && (ctx->tc & RISCV_IOMMU_DC_TC_V)) {

-        *ref = ctx_cache;
-        return ctx;
+    if (mode != RISCV_IOMMU_DDTP_MODE_OFF &&
+        mode != RISCV_IOMMU_DDTP_MODE_BARE) {
+        ctx = g_hash_table_lookup(ctx_cache, &key);
+
+        if (ctx && (ctx->tc & RISCV_IOMMU_DC_TC_V)) {
+            *ref = ctx_cache;
+            return ctx;
+        }
      }

ctx = g_new0(RISCVIOMMUContext, 1);