[XEN PATCH v4 4/5] VT-d: Disable intrerrupt remapping if cx16 is not supported

[Teddy Astie <teddy.astie@vates.tech>]

All hardware with VT-d has CMPXCHG16B support.  Check this at initialisation
time, and remove the effectively-dead logic for the non-cx16 case.

Suggested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Teddy Astie <teddy.astie@vates.tech>
---
 xen/drivers/passthrough/vtd/intremap.c | 70 ++++++++------------------
 xen/drivers/passthrough/vtd/iommu.c    |  7 +--
 2 files changed, 21 insertions(+), 56 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/intremap.c b/xen/drivers/passthrough/vtd/intremap.c
index c504852eb8..7d4d907b4f 100644
--- a/xen/drivers/passthrough/vtd/intremap.c
+++ b/xen/drivers/passthrough/vtd/intremap.c
@@ -150,6 +150,13 @@ bool __init cf_check intel_iommu_supports_eim(void)
     if ( !iommu_qinval || !iommu_intremap || list_empty(&acpi_drhd_units) )
         return false;
 
+    if ( unlikely(!cpu_has_cx16) )
+    {
+        printk(XENLOG_WARNING VTDPREFIX
+               "CPU doesn't support CMPXCHG16B, disable interrupt remapping\n");
+        return false;
+    }
+
     /* We MUST have a DRHD unit for each IOAPIC. */
     for ( apic = 0; apic < nr_ioapics; apic++ )
         if ( !ioapic_to_drhd(IO_APIC_ID(apic)) )
@@ -173,47 +180,26 @@ 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));
+    __uint128_t ret;
+    struct iremap_entry old_ire;
 
-    if ( cpu_has_cx16 )
-    {
-        __uint128_t ret;
-        struct iremap_entry old_ire;
+    ASSERT(spin_is_locked(&iommu->intremap.lock));
 
-        old_ire = *entry;
-        ret = cmpxchg16b(entry, &old_ire, new_ire);
+    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 */
@@ -395,7 +381,6 @@ static int ioapic_rte_to_remap_entry(struct vtd_iommu *iommu,
     /* 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 +422,6 @@ 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 7c6bae0256..a1bd3c5ff6 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -2663,12 +2663,7 @@ 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