No hardware has VT-d support while not having cx16 support, consider disabling IOMMU in this case to avoid potentially buggy code.

Now that IOMMU is only enabled if cx16 is supported, drop dead code that handles cases where cx16 isn't supported.

Signed-off-by Teddy Astie <teddy.astie@vates.tech>
---
 xen/drivers/passthrough/amd/iommu_map.c     | 43 +++++++--------------
 xen/drivers/passthrough/amd/pci_amd_iommu.c |  6 +++
 2 files changed, 21 insertions(+), 28 deletions(-)

diff --git a/xen/drivers/passthrough/amd/iommu_map.c b/xen/drivers/passthrough/amd/iommu_map.c
index e0f4fe736a..c8c1c0cfae 100644
--- a/xen/drivers/passthrough/amd/iommu_map.c
+++ b/xen/drivers/passthrough/amd/iommu_map.c
@@ -167,15 +167,14 @@ int amd_iommu_set_root_page_table(struct amd_iommu_dte *dte,
 {
     bool valid = flags & SET_ROOT_VALID;
 
-    if ( dte->v && dte->tv &&
-         (cpu_has_cx16 || (flags & SET_ROOT_WITH_UNITY_MAP)) )
+    if ( dte->v && dte->tv )
     {
         union {
             struct amd_iommu_dte dte;
             uint64_t raw64[4];
             __uint128_t raw128[2];
         } ldte = { .dte = *dte };
-        __uint128_t old = ldte.raw128[0];
+        __uint128_t res, old = ldte.raw128[0];
         int ret = 0;
 
         ldte.dte.domain_id = domain_id;
@@ -185,33 +184,21 @@ int amd_iommu_set_root_page_table(struct amd_iommu_dte *dte,
         ldte.dte.paging_mode = paging_mode;
         ldte.dte.v = valid;
 
-        if ( cpu_has_cx16 )
-        {
-            __uint128_t res = cmpxchg16b(dte, &old, &ldte.raw128[0]);
+    
+        res = cmpxchg16b(dte, &old, &ldte.raw128[0]);
 
-            /*
-             * Hardware does not update the DTE behind our backs, so the
-             * return value should match "old".
-             */
-            if ( res != old )
-            {
-                printk(XENLOG_ERR
-                       "Dom%d: unexpected DTE %016lx_%016lx (expected %016lx_%016lx)\n",
-                       domain_id,
-                       (uint64_t)(res >> 64), (uint64_t)res,
-                       (uint64_t)(old >> 64), (uint64_t)old);
-                ret = -EILSEQ;
-            }
-        }
-        else /* Best effort, updating domain_id last. */
+        /*
+         * Hardware does not update the DTE behind our backs, so the
+         * return value should match "old".
+         */
+        if ( res != old )
         {
-            uint64_t *ptr = (void *)dte;
-
-            write_atomic(ptr + 0, ldte.raw64[0]);
-            /* No barrier should be needed between these two. */
-            write_atomic(ptr + 1, ldte.raw64[1]);
-
-            ret = 1;
+            printk(XENLOG_ERR
+                   "Dom%d: unexpected DTE %016lx_%016lx (expected %016lx_%016lx)\n",
+                   domain_id,
+                   (uint64_t)(res >> 64), (uint64_t)res,
+                   (uint64_t)(old >> 64), (uint64_t)old);
+            ret = -EILSEQ;
         }
 
         return ret;
diff --git a/xen/drivers/passthrough/amd/pci_amd_iommu.c b/xen/drivers/passthrough/amd/pci_amd_iommu.c
index f6efd88e36..656c5eda5d 100644
--- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
+++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
@@ -312,6 +312,12 @@ static int __init cf_check iov_detect(void)
         return -ENODEV;
     }
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk("AMD-Vi: CPU doesn't support CMPXCHG16B, disabling\n");
+        return -ENOSYS;
+    }
+
     init_done = 1;
 
     if ( !amd_iommu_perdev_intremap )
-- 
2.44.0



Teddy Astie | Vates XCP-ng Intern

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech

On 21.03.2024 11:28, Teddy Astie wrote:
> No hardware has VT-d support while not having cx16 support, consider disabling IOMMU in this case to avoid potentially buggy code.

VT-d? That's Intel, not AMD. Also alongside bare hardware you also want to
not completely leave out Xen running virtualized itself.

As a nit, please limit description lines to 80 chars.

> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> @@ -312,6 +312,12 @@ static int __init cf_check iov_detect(void)
>          return -ENODEV;
>      }
>  
> +    if ( unlikely(!cpu_has_cx16) )
> +    {
> +        printk("AMD-Vi: CPU doesn't support CMPXCHG16B, disabling\n");
> +        return -ENOSYS;

ENOSYS has a dedicated purpose and hence doesn't fit here. If no better
suitable error code can be identified, ENODEV is perhaps what ought to be
used.

I'd also like to note that while this says 2/3, I can't spot any other
part of such a 3-patch series anywhere. However, in particular
https://lists.xen.org/archives/html/xen-devel/2024-03/threads.html looks
to be lagging anyway.

Further please note that maintainership recently has changed some; please
make sure you use up-to-date information for composing the Cc: list.

Jan