amd_iommu: Return an error when unable to read PTE from guest memory

Make amdvi_get_pte_entry() return an error value (-1) in cases where the
memory read fails, versus the current return of 0 to indicate failure.
The reason is that 0 is also a valid value to have stored in the PTE in
guest memory i.e. the guest does not have a mapping. Before this change,
amdvi_get_pte_entry() returned 0 for both an error and for empty PTEs, but
the page walker implementation that will be introduced in upcoming changes
needs a method to differentiate between the two scenarios.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20250919213515.917111-7-alejandro.j.jimenez@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/hw/i386/amd_iommu.c b/hw/i386/amd_iommu.c
index dc7531fd4a..29ed3f0ef2 100644
--- a/hw/i386/amd_iommu.c
+++ b/hw/i386/amd_iommu.c
@@ -528,7 +528,7 @@ static inline uint64_t amdvi_get_pte_entry(AMDVIState *s, uint64_t pte_addr,
                         &pte, sizeof(pte), MEMTXATTRS_UNSPECIFIED)) {
         trace_amdvi_get_pte_hwerror(pte_addr);
         amdvi_log_pagetab_error(s, devid, pte_addr, 0);
-        pte = 0;
+        pte = (uint64_t)-1;
         return pte;
     }
 
@@ -1081,7 +1081,7 @@ static void amdvi_page_walk(AMDVIAddressSpace *as, uint64_t *dte,
             /* add offset and load pte */
             pte_addr += ((addr >> (3 + 9 * level)) & 0x1FF) << 3;
             pte = amdvi_get_pte_entry(as->iommu_state, pte_addr, as->devfn);
-            if (!pte) {
+            if (!pte || (pte == (uint64_t)-1)) {
                 return;
             }
             oldlevel = level;