[PATCH v2 1/2] iommu/tegra: gart: Optionally check for overwriting of page mappings

[Dmitry Osipenko]

Due to a bug in IOVA allocator, page mapping could accidentally overwritten.
We can catch this case by checking 'VALID' bit of GART's page entry prior to
mapping of a page. Since that check introduces a noticeable performance
impact, it should be enabled explicitly by a new CONFIG_TEGRA_IOMMU_GART_DEBUG
option.

Signed-off-by: Dmitry Osipenko <dig...@gmail.com>
---
 drivers/iommu/Kconfig      |  9 +++++++++
 drivers/iommu/tegra-gart.c | 16 +++++++++++++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/Kconfig b/drivers/iommu/Kconfig
index f3a21343e636..851156a4896d 100644
--- a/drivers/iommu/Kconfig
+++ b/drivers/iommu/Kconfig
@@ -242,6 +242,15 @@ config TEGRA_IOMMU_GART
          space through the GART (Graphics Address Relocation Table)
          hardware included on Tegra SoCs.
 
+config TEGRA_IOMMU_GART_DEBUG
+       bool "Debug Tegra GART IOMMU"
+       depends on TEGRA_IOMMU_GART
+       help
+         Properly unmap pages and check whether page is already mapped
+         during of mapping in expense of performance. This allows to
+         catch double page remappings, caused by a bug in the IOVA
+         allocator for example.
+
 config TEGRA_IOMMU_SMMU
        bool "NVIDIA Tegra SMMU Support"
        depends on ARCH_TEGRA
diff --git a/drivers/iommu/tegra-gart.c b/drivers/iommu/tegra-gart.c
index b62f790ad1ba..bc4cb200fa03 100644
--- a/drivers/iommu/tegra-gart.c
+++ b/drivers/iommu/tegra-gart.c
@@ -271,6 +271,7 @@ static int gart_iommu_map(struct iommu_domain *domain, 
unsigned long iova,
        struct gart_device *gart = gart_domain->gart;
        unsigned long flags;
        unsigned long pfn;
+       unsigned long pte;
 
        if (!gart_iova_range_valid(gart, iova, bytes))
                return -EINVAL;
@@ -282,6 +283,14 @@ static int gart_iommu_map(struct iommu_domain *domain, 
unsigned long iova,
                spin_unlock_irqrestore(&gart->pte_lock, flags);
                return -EINVAL;
        }
+       if (IS_ENABLED(TEGRA_IOMMU_GART_DEBUG)) {
+               pte = gart_read_pte(gart, iova);
+               if (pte & GART_ENTRY_PHYS_ADDR_VALID) {
+                       spin_unlock_irqrestore(&gart->pte_lock, flags);
+                       dev_err(gart->dev, "Page entry is used already\n");
+                       return -EBUSY;
+               }
+       }
        gart_set_pte(gart, iova, GART_PTE(pfn));
        FLUSH_GART_REGS(gart);
        spin_unlock_irqrestore(&gart->pte_lock, flags);
@@ -295,6 +304,10 @@ static size_t gart_iommu_unmap(struct iommu_domain 
*domain, unsigned long iova,
        struct gart_device *gart = gart_domain->gart;
        unsigned long flags;
 
+       /* don't unmap page entries to achieve better performance */
+       if (!IS_ENABLED(TEGRA_IOMMU_GART_DEBUG))
+               return 0;
+
        if (!gart_iova_range_valid(gart, iova, bytes))
                return 0;
 
@@ -302,7 +315,8 @@ static size_t gart_iommu_unmap(struct iommu_domain *domain, 
unsigned long iova,
        gart_set_pte(gart, iova, 0);
        FLUSH_GART_REGS(gart);
        spin_unlock_irqrestore(&gart->pte_lock, flags);
-       return 0;
+
+       return bytes;
 }
 
 static phys_addr_t gart_iommu_iova_to_phys(struct iommu_domain *domain,
-- 
2.14.1

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