Re: [PATCH] hw/riscv/riscv-iommu: fix U-bit check to apply only to leaf S/VS-stage PTEs

On Fri, 2026-07-17 at 13:23 +0200, Andrew Jones wrote:
> Commit b795ea0ba471 ("hw/riscv/riscv-iommu.c: fault when !PTE_U and
> no priv access") placed its check ahead of the leaf-vs-non-leaf
> branch in riscv_iommu_spa_fetch(), so it fires on every PTE walked,
> including non-leaf/table entries. Per the RISC-V privileged spec's
> address translation algorithm (Sv39/Sv48/etc., the "leaf PTE has
> been reached" step, followed separately by the U-bit permission
> check), the U bit is only defined and checked for the leaf PTE
> reached at the end of the walk -- non-leaf PTEs don't carry a
> meaningful U bit at all.
> 
> Move the check after the leaf/non-leaf branch, alongside the other
> leaf-only checks, mirroring how the G_STAGE U-bit check (added in
> 9158c900ab30) is already correctly placed.
> 
> Fixes: b795ea0ba471 ("hw/riscv/riscv-iommu.c: fault when !PTE_U and
> no priv access")
> Signed-off-by: Andrew Jones <[email protected]>

Thanks!

Applied to riscv-to-apply.next

Alistair

> ---
>  hw/riscv/riscv-iommu.c | 14 +++++++-------
>  1 file changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/hw/riscv/riscv-iommu.c b/hw/riscv/riscv-iommu.c
> index f6865d1e164e..ed9fb09f8bc7 100644
> --- a/hw/riscv/riscv-iommu.c
> +++ b/hw/riscv/riscv-iommu.c
> @@ -476,13 +476,6 @@ static int riscv_iommu_spa_fetch(RISCVIOMMUState
> *s, RISCVIOMMUContext *ctx,
>              break;                /* Invalid PTE */
>          } else if (pte & PTE_RESERVED(false)) {
>              break;                /* Reserved PTE bits set */
> -        } else if (!(pte & PTE_U) && !pv) {
> -            /*
> -             * All accesses are assumed to be User mode unless
> -             * process_id is valid (pv).  In case we have a
> -             * non-user mode PTE and !pv we need to fault.
> -             */
> -            break;
>          } else if (!(pte & (PTE_R | PTE_W | PTE_X))) {
>              base = PPN_PHYS(ppn); /* Inner PTE, continue walking */
>          } else if ((pte & (PTE_R | PTE_W | PTE_X)) == PTE_W) {
> @@ -491,6 +484,13 @@ static int riscv_iommu_spa_fetch(RISCVIOMMUState
> *s, RISCVIOMMUContext *ctx,
>              break;                /* Reserved leaf PTE flags: PTE_W
> + PTE_X */
>          } else if (ppn & ((1ULL << (va_skip - TARGET_PAGE_BITS)) -
> 1)) {
>              break;                /* Misaligned PPN */
> +        } else if (!(pte & PTE_U) && !pv) {
> +            /*
> +             * All accesses are assumed to be User mode unless
> +             * process_id is valid (pv).  In case we have a
> +             * non-user mode leaf PTE and !pv we need to fault.
> +             */
> +            break;
>          } else if ((iotlb->perm & IOMMU_RO) && !(pte & PTE_R)) {
>              break;                /* Read access check failed */
>          } else if ((iotlb->perm & IOMMU_WO) && !(pte & PTE_W)) {