[XEN PATCH 0/3] x86/iommu: Drop IOMMU support when cpu doesn't support cx16.

[XEN PATCH 0/3] x86/iommu: Drop IOMMU support when cpu doesn't support cx16.

[Teddy Astie]

All hardware that supports VT-d/AMD-Vi that exists also supports cx16 (aside specifically crafted virtual machines).

Some IOMMU code paths in Xen consider cases where VT-d/AMD-Vi is supported while cx16 isn't, those paths may be bugged and are barely tested, dead code in practice.

Consider disabling IOMMU in case we have IOMMU hardware but no cx16, then cleanup no-cx16 handling logic from VT-d and AMD-Vi drivers.

Teddy Astie (3):
  VT-d: Disable IOMMU if cx16 isn't supported
  AMD-Vi: Disable IOMMU if cx16 isn't supported
  VT-d: Cleanup MAP_SINGLE_DEVICE and related dead code.

 xen/drivers/passthrough/amd/iommu_map.c     | 43 ++++------
 xen/drivers/passthrough/amd/pci_amd_iommu.c |  6 ++
 xen/drivers/passthrough/vtd/intremap.c      | 67 ++++-----------
 xen/drivers/passthrough/vtd/iommu.c         | 92 +++++++--------------
 xen/drivers/passthrough/vtd/vtd.h           |  5 +-
 5 files changed, 67 insertions(+), 146 deletions(-)

-- 
2.44.0



Teddy Astie | Vates XCP-ng Intern

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech

[XEN PATCH 1/3] VT-d: Disable IOMMU if cx16 isn't supported

[Teddy Astie]

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/vtd/intremap.c | 67 +++++----------------
 xen/drivers/passthrough/vtd/iommu.c    | 80 +++++++++-----------------
 2 files changed, 41 insertions(+), 106 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/intremap.c b/xen/drivers/passthrough/vtd/intremap.c
index c504852eb8..312b73e693 100644
--- a/xen/drivers/passthrough/vtd/intremap.c
+++ b/xen/drivers/passthrough/vtd/intremap.c
@@ -173,47 +173,26 @@ bool __init cf_check intel_iommu_supports_eim(void)
  * Assume iremap_lock has been acquired. It is to make sure software will not
  * change the same IRTE behind us. With this assumption, if only high qword or
  * low qword in IRTE is to be updated, this function's atomic variant can
- * present an atomic update to VT-d hardware even when cmpxchg16b
- * instruction is not supported.
+ * present an atomic update to VT-d hardware.
  */
 static void update_irte(struct vtd_iommu *iommu, struct iremap_entry *entry,
                         const struct iremap_entry *new_ire, bool atomic)
 {
-    ASSERT(spin_is_locked(&iommu->intremap.lock));
-
-    if ( cpu_has_cx16 )
-    {
-        __uint128_t ret;
-        struct iremap_entry old_ire;
+    __uint128_t ret;
+    struct iremap_entry old_ire;
 
-        old_ire = *entry;
-        ret = cmpxchg16b(entry, &old_ire, new_ire);
+    ASSERT(spin_is_locked(&iommu->intremap.lock));
+    
+    old_ire = *entry;
+    ret = cmpxchg16b(entry, &old_ire, new_ire);
 
-        /*
-         * In the above, we use cmpxchg16 to atomically update the 128-bit
-         * IRTE, and the hardware cannot update the IRTE behind us, so
-         * the return value of cmpxchg16 should be the same as old_ire.
-         * This ASSERT validate it.
-         */
-        ASSERT(ret == old_ire.val);
-    }
-    else
-    {
-        /*
-         * VT-d hardware doesn't update IRTEs behind us, nor the software
-         * since we hold iremap_lock. If the caller wants VT-d hardware to
-         * always see a consistent entry, but we can't meet it, a bug will
-         * be raised.
-         */
-        if ( entry->lo == new_ire->lo )
-            write_atomic(&entry->hi, new_ire->hi);
-        else if ( entry->hi == new_ire->hi )
-            write_atomic(&entry->lo, new_ire->lo);
-        else if ( !atomic )
-            *entry = *new_ire;
-        else
-            BUG();
-    }
+    /*
+     * In the above, we use cmpxchg16 to atomically update the 128-bit
+     * IRTE, and the hardware cannot update the IRTE behind us, so
+     * the return value of cmpxchg16 should be the same as old_ire.
+     * This ASSERT validate it.
+     */
+    ASSERT(ret == old_ire.val);
 }
 
 /* Mark specified intr remap entry as free */
@@ -394,8 +373,7 @@ static int ioapic_rte_to_remap_entry(struct vtd_iommu *iommu,
     remap_rte->reserved = 0;
     /* Indicate remap format. */
     remap_rte->format = 1;
-
-    /* If cmpxchg16b is not available the caller must mask the IO-APIC pin. */
+    
     update_irte(iommu, iremap_entry, &new_ire, !init && !masked);
     iommu_sync_cache(iremap_entry, sizeof(*iremap_entry));
     iommu_flush_iec_index(iommu, 0, index);
@@ -437,21 +415,6 @@ void cf_check io_apic_write_remap_rte(
     bool masked = true;
     int rc;
 
-    if ( !cpu_has_cx16 )
-    {
-       /*
-        * Cannot atomically update the IRTE entry: mask the IO-APIC pin to
-        * avoid interrupts seeing an inconsistent IRTE entry.
-        */
-        old_rte = __ioapic_read_entry(apic, pin, true);
-        if ( !old_rte.mask )
-        {
-            masked = false;
-            old_rte.mask = 1;
-            __ioapic_write_entry(apic, pin, true, old_rte);
-        }
-    }
-
     /* Not the initializer, for old gcc to cope. */
     new_rte.raw = rte;
 
diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index c7110af7c9..47b56f37a9 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1482,7 +1482,7 @@ int domain_context_mapping_one(
 {
     struct domain_iommu *hd = dom_iommu(domain);
     struct context_entry *context, *context_entries, lctxt;
-    __uint128_t old;
+    __uint128_t res, old;
     uint64_t maddr;
     uint16_t seg = iommu->drhd->segment, prev_did = 0;
     struct domain *prev_dom = NULL;
@@ -1580,55 +1580,23 @@ int domain_context_mapping_one(
         ASSERT(!context_fault_disable(lctxt));
     }
 
-    if ( cpu_has_cx16 )
-    {
-        __uint128_t res = cmpxchg16b(context, &old, &lctxt.full);
+    res = cmpxchg16b(context, &old, &lctxt.full);
 
-        /*
-         * Hardware does not update the context entry behind our backs,
-         * so the return value should match "old".
-         */
-        if ( res != old )
-        {
-            if ( pdev )
-                check_cleanup_domid_map(domain, pdev, iommu);
-            printk(XENLOG_ERR
-                   "%pp: unexpected context entry %016lx_%016lx (expected %016lx_%016lx)\n",
-                   &PCI_SBDF(seg, bus, devfn),
-                   (uint64_t)(res >> 64), (uint64_t)res,
-                   (uint64_t)(old >> 64), (uint64_t)old);
-            rc = -EILSEQ;
-            goto unlock;
-        }
-    }
-    else if ( !prev_dom || !(mode & MAP_WITH_RMRR) )
-    {
-        context_clear_present(*context);
-        iommu_sync_cache(context, sizeof(*context));
-
-        write_atomic(&context->hi, lctxt.hi);
-        /* No barrier should be needed between these two. */
-        write_atomic(&context->lo, lctxt.lo);
-    }
-    else /* Best effort, updating DID last. */
+    /*
+     * Hardware does not update the context entry behind our backs,
+     * so the return value should match "old".
+     */
+    if ( res != old )
     {
-         /*
-          * By non-atomically updating the context entry's DID field last,
-          * during a short window in time TLB entries with the old domain ID
-          * but the new page tables may be inserted.  This could affect I/O
-          * of other devices using this same (old) domain ID.  Such updating
-          * therefore is not a problem if this was the only device associated
-          * with the old domain ID.  Diverting I/O of any of a dying domain's
-          * devices to the quarantine page tables is intended anyway.
-          */
-        if ( !(mode & (MAP_OWNER_DYING | MAP_SINGLE_DEVICE)) )
-            printk(XENLOG_WARNING VTDPREFIX
-                   " %pp: reassignment may cause %pd data corruption\n",
-                   &PCI_SBDF(seg, bus, devfn), prev_dom);
-
-        write_atomic(&context->lo, lctxt.lo);
-        /* No barrier should be needed between these two. */
-        write_atomic(&context->hi, lctxt.hi);
+        if ( pdev )
+            check_cleanup_domid_map(domain, pdev, iommu);
+        printk(XENLOG_ERR
+                "%pp: unexpected context entry %016lx_%016lx (expected %016lx_%016lx)\n",
+                &PCI_SBDF(seg, bus, devfn),
+                (uint64_t)(res >> 64), (uint64_t)res,
+                (uint64_t)(old >> 64), (uint64_t)old);
+        rc = -EILSEQ;
+        goto unlock;
     }
 
     iommu_sync_cache(context, sizeof(struct context_entry));
@@ -2630,6 +2598,15 @@ static int __init cf_check vtd_setup(void)
     int ret;
     bool reg_inval_supported = true;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_ERR VTDPREFIX
+               "IOMMU: CPU doesn't support CMPXCHG16B, disabling\n");
+
+        ret = -ENOSYS;
+        goto error;
+    }
+
     if ( list_empty(&acpi_drhd_units) )
     {
         ret = -ENODEV;
@@ -2692,12 +2669,7 @@ static int __init cf_check vtd_setup(void)
             iommu_intremap = iommu_intremap_off;
 
 #ifndef iommu_intpost
-        /*
-         * We cannot use posted interrupt if X86_FEATURE_CX16 is
-         * not supported, since we count on this feature to
-         * atomically update 16-byte IRTE in posted format.
-         */
-        if ( !cap_intr_post(iommu->cap) || !iommu_intremap || !cpu_has_cx16 )
+        if ( !cap_intr_post(iommu->cap) || !iommu_intremap )
             iommu_intpost = false;
 #endif
 
-- 
2.44.0



Teddy Astie | Vates XCP-ng Intern

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech

[XEN PATCH 3/3] VT-d: Cleanup MAP_SINGLE_DEVICE and related dead code.

[Teddy Astie]

This flag was only used in case cx16 is not available, as those code paths no longer exist, this flag now does basically nothing.

Signed-off-by Teddy Astie <teddy.astie@vates.tech>
---
 xen/drivers/passthrough/vtd/iommu.c | 12 +++---------
 xen/drivers/passthrough/vtd/vtd.h   |  5 ++---
 2 files changed, 5 insertions(+), 12 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index 47b56f37a9..4b15e6da79 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -1692,15 +1692,9 @@ static int domain_context_mapping(struct domain *domain, u8 devfn,
         break;
     }
 
-    if ( domain != pdev->domain && pdev->domain != dom_io )
-    {
-        if ( pdev->domain->is_dying )
-            mode |= MAP_OWNER_DYING;
-        else if ( drhd &&
-                  !any_pdev_behind_iommu(pdev->domain, pdev, drhd->iommu) &&
-                  !pdev->phantom_stride )
-            mode |= MAP_SINGLE_DEVICE;
-    }
+    if ( domain != pdev->domain && pdev->domain != dom_io &&
+         pdev->domain->is_dying )
+        mode |= MAP_OWNER_DYING;
 
     switch ( pdev->type )
     {
diff --git a/xen/drivers/passthrough/vtd/vtd.h b/xen/drivers/passthrough/vtd/vtd.h
index cb2df76eed..43f06a353d 100644
--- a/xen/drivers/passthrough/vtd/vtd.h
+++ b/xen/drivers/passthrough/vtd/vtd.h
@@ -28,9 +28,8 @@
  */
 #define MAP_WITH_RMRR         (1u << 0)
 #define MAP_OWNER_DYING       (1u << 1)
-#define MAP_SINGLE_DEVICE     (1u << 2)
-#define MAP_ERROR_RECOVERY    (1u << 3)
-#define UNMAP_ME_PHANTOM_FUNC (1u << 4)
+#define MAP_ERROR_RECOVERY    (1u << 2)
+#define UNMAP_ME_PHANTOM_FUNC (1u << 3)
 
 /* Allow for both IOAPIC and IOSAPIC. */
 #define IO_xAPIC_route_entry IO_APIC_route_entry
-- 
2.44.0



Teddy Astie | Vates XCP-ng Intern

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech

[XEN PATCH 2/3] AMD-Vi: Disable IOMMU if cx16 isn't supported

[Teddy Astie]

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

Re: [XEN PATCH 2/3] AMD-Vi: Disable IOMMU if cx16 isn't supported

[Jan Beulich]

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

Re: [XEN PATCH 1/3] VT-d: Disable IOMMU if cx16 isn't supported

[Jan Beulich]

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.

Like in patch 2 (which for whatever reason made it through quite a bit earlier),
why "consider"? Your change does disable the IOMMU in that case, you don't merely
consider doing so.

> @@ -394,8 +373,7 @@ static int ioapic_rte_to_remap_entry(struct vtd_iommu *iommu,
>      remap_rte->reserved = 0;
>      /* Indicate remap format. */
>      remap_rte->format = 1;
> -
> -    /* If cmpxchg16b is not available the caller must mask the IO-APIC pin. */
> +    

Please don't introduce lines with trailing blanks.

> @@ -2630,6 +2598,15 @@ static int __init cf_check vtd_setup(void)
>      int ret;
>      bool reg_inval_supported = true;
>  
> +    if ( unlikely(!cpu_has_cx16) )
> +    {
> +        printk(XENLOG_ERR VTDPREFIX
> +               "IOMMU: CPU doesn't support CMPXCHG16B, disabling\n");
> +
> +        ret = -ENOSYS;
> +        goto error;
> +    }

One thing I forgot to mention when replying to patch 2, applying here
equally: vtd_setup() isn't necessarily the first bit of IOMMU setup done.
For x2APIC enabling earlier calls out of x2apic_bsp_setup() exist, which
also need taking care of.

Jan