[PATCH] iommupt: Avoid a compiler bug with sw_bit

[PATCH] iommupt: Avoid a compiler bug with sw_bit

[Jason Gunthorpe]

gcc 13, in some cases, gets confused if the __builtin_constant_p() is
inside the switch. It thinks that bitnr can have the value max+1 and
fails. Lift the check outside the switch to avoid it.

Fixes: ef7bfe5bbffd ("iommupt/x86: Support SW bits and permit PT_FEAT_DMA_INCOHERENT")
Fixes: 5448c1558f60 ("iommupt: Add the Intel VT-d second stage page table format")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511242012.I7g504Ab-lkp@intel.com/
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
---
 drivers/iommu/generic_pt/fmt/vtdss.h  | 8 ++++----
 drivers/iommu/generic_pt/fmt/x86_64.h | 8 ++++----
 2 files changed, 8 insertions(+), 8 deletions(-)

Seems to need a specific randconfig to fail, my normal ones don't, nor has 0
day found this on its randconfigs till now.

diff --git a/drivers/iommu/generic_pt/fmt/vtdss.h b/drivers/iommu/generic_pt/fmt/vtdss.h
index d9774848eb6f7f..50ffed9d0e508f 100644
--- a/drivers/iommu/generic_pt/fmt/vtdss.h
+++ b/drivers/iommu/generic_pt/fmt/vtdss.h
@@ -174,6 +174,9 @@ static inline unsigned int vtdss_pt_max_sw_bit(struct pt_common *common)
 
 static inline u64 vtdss_pt_sw_bit(unsigned int bitnr)
 {
+	if (__builtin_constant_p(bitnr) && bitnr > 10)
+		BUILD_BUG();
+
 	/* Bits marked Ignored in the specification */
 	switch (bitnr) {
 	case 0:
@@ -184,10 +187,7 @@ static inline u64 vtdss_pt_sw_bit(unsigned int bitnr)
 		return BIT_ULL(63);
 	/* Some bits in 9-3 are available in some entries */
 	default:
-		if (__builtin_constant_p(bitnr))
-			BUILD_BUG();
-		else
-			PT_WARN_ON(true);
+		PT_WARN_ON(true);
 		return 0;
 	}
 }
diff --git a/drivers/iommu/generic_pt/fmt/x86_64.h b/drivers/iommu/generic_pt/fmt/x86_64.h
index a86353f1481e5c..507abf2c934ccb 100644
--- a/drivers/iommu/generic_pt/fmt/x86_64.h
+++ b/drivers/iommu/generic_pt/fmt/x86_64.h
@@ -175,6 +175,9 @@ static inline unsigned int x86_64_pt_max_sw_bit(struct pt_common *common)
 
 static inline u64 x86_64_pt_sw_bit(unsigned int bitnr)
 {
+	if (__builtin_constant_p(bitnr) && bitnr > 12)
+		BUILD_BUG();
+
 	/* Bits marked Ignored/AVL in the specification */
 	switch (bitnr) {
 	case 0:
@@ -185,10 +188,7 @@ static inline u64 x86_64_pt_sw_bit(unsigned int bitnr)
 		return BIT_ULL((bitnr - 2) + 52);
 	/* Some bits in 8,6,4,3 are available in some entries */
 	default:
-		if (__builtin_constant_p(bitnr))
-			BUILD_BUG();
-		else
-			PT_WARN_ON(true);
+		PT_WARN_ON(true);
 		return 0;
 	}
 }

base-commit: 93d0836a0e54f678a3814cf07fa1339fe6cce4d7
-- 
2.43.0

Re: [PATCH] iommupt: Avoid a compiler bug with sw_bit

[Joerg Roedel]

On Wed, Nov 26, 2025 at 03:21:27PM -0400, Jason Gunthorpe wrote:
> gcc 13, in some cases, gets confused if the __builtin_constant_p() is
> inside the switch. It thinks that bitnr can have the value max+1 and
> fails. Lift the check outside the switch to avoid it.
> 
> Fixes: ef7bfe5bbffd ("iommupt/x86: Support SW bits and permit PT_FEAT_DMA_INCOHERENT")
> Fixes: 5448c1558f60 ("iommupt: Add the Intel VT-d second stage page table format")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202511242012.I7g504Ab-lkp@intel.com/
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>  drivers/iommu/generic_pt/fmt/vtdss.h  | 8 ++++----
>  drivers/iommu/generic_pt/fmt/x86_64.h | 8 ++++----
>  2 files changed, 8 insertions(+), 8 deletions(-)

Applied, thanks.