diff --git a/xen/drivers/passthrough/vtd/dmar.h b/xen/drivers/passthrough/vtd/dmar.h
index 1a9c965e59..a44820a5e7 100644
--- a/xen/drivers/passthrough/vtd/dmar.h
+++ b/xen/drivers/passthrough/vtd/dmar.h
@@ -127,6 +127,34 @@ do {                                                \
     }                                                           \
 } while (0)
 
+#define IOMMU_FLUSH_WAIT(what, iommu, offset, op, cond, sts)       \
+do {                                                               \
+    static unsigned int __read_mostly threshold = 1;               \
+    s_time_t start = NOW();                                        \
+    s_time_t timeout = start + DMAR_OPERATION_TIMEOUT * threshold; \
+                                                                   \
+    for ( ; ; )                                                    \
+    {                                                              \
+        sts = op(iommu->reg, offset);                              \
+        if ( cond )                                                \
+            break;                                                 \
+        if ( timeout && NOW() > timeout )                          \
+        {                                                          \
+            threshold |= threshold << 1;                           \
+            printk(XENLOG_WARNING VTDPREFIX                        \
+                   " IOMMU#%u: %s flush taking too long\n",        \
+                   iommu->index, what);                            \
+            timeout = 0;                                           \
+        }                                                          \
+        cpu_relax();                                               \
+    }                                                              \
+                                                                   \
+    if ( !timeout )                                                \
+        printk(XENLOG_WARNING VTDPREFIX                            \
+               " IOMMU#%u: %s flush took %lums\n",                 \
+               iommu->index, what, (NOW() - start) / 10000000);    \
+} while ( false )
+
 int vtd_hw_check(void);
 void disable_pmr(struct vtd_iommu *iommu);
 int is_igd_drhd(struct acpi_drhd_unit *drhd);
diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index 34ee5d22f1..dfd867d40b 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -320,8 +320,8 @@ static void iommu_flush_write_buffer(struct vtd_iommu *iommu)
     dmar_writel(iommu->reg, DMAR_GCMD_REG, val | DMA_GCMD_WBF);
 
     /* Make sure hardware complete it */
-    IOMMU_WAIT_OP(iommu, DMAR_GSTS_REG, dmar_readl,
-                  !(val & DMA_GSTS_WBFS), val);
+    IOMMU_FLUSH_WAIT("write buffer", iommu, DMAR_GSTS_REG, dmar_readl,
+                     !(val & DMA_GSTS_WBFS), val);
 
     spin_unlock_irqrestore(&iommu->register_lock, flags);
 }
@@ -370,8 +370,8 @@ int vtd_flush_context_reg(struct vtd_iommu *iommu, uint16_t did,
     dmar_writeq(iommu->reg, DMAR_CCMD_REG, val);
 
     /* Make sure hardware complete it */
-    IOMMU_WAIT_OP(iommu, DMAR_CCMD_REG, dmar_readq,
-                  !(val & DMA_CCMD_ICC), val);
+    IOMMU_FLUSH_WAIT("context", iommu, DMAR_CCMD_REG, dmar_readq,
+                     !(val & DMA_CCMD_ICC), val);
 
     spin_unlock_irqrestore(&iommu->register_lock, flags);
     /* flush context entry will implicitly flush write buffer */
@@ -448,8 +448,8 @@ int vtd_flush_iotlb_reg(struct vtd_iommu *iommu, uint16_t did, uint64_t addr,
     dmar_writeq(iommu->reg, tlb_offset + 8, val);
 
     /* Make sure hardware complete it */
-    IOMMU_WAIT_OP(iommu, (tlb_offset + 8), dmar_readq,
-                  !(val & DMA_TLB_IVT), val);
+    IOMMU_FLUSH_WAIT("iotlb", iommu, (tlb_offset + 8), dmar_readq,
+                     !(val & DMA_TLB_IVT), val);
     spin_unlock_irqrestore(&iommu->register_lock, flags);
 
     /* check IOTLB invalidation granularity */
diff --git a/xen/drivers/passthrough/vtd/qinval.c b/xen/drivers/passthrough/vtd/qinval.c
index 86a941ce27..1056ab1feb 100644
--- a/xen/drivers/passthrough/vtd/qinval.c
+++ b/xen/drivers/passthrough/vtd/qinval.c
@@ -29,8 +29,6 @@
 #include "extern.h"
 #include "../ats.h"
 
-#define VTD_QI_TIMEOUT	1
-
 static unsigned int __read_mostly qi_pg_order;
 static unsigned int __read_mostly qi_entry_nr;
 
@@ -60,7 +58,11 @@ static unsigned int qinval_next_index(struct vtd_iommu *iommu)
     /* (tail+1 == head) indicates a full queue, wait for HW */
     while ( ((tail + 1) & (qi_entry_nr - 1)) ==
             ( dmar_readq(iommu->reg, DMAR_IQH_REG) >> QINVAL_INDEX_SHIFT ) )
+    {
+        printk_once(XENLOG_ERR VTDPREFIX " IOMMU#%u: no QI slot available\n",
+                    iommu->index);
         cpu_relax();
+    }
 
     return tail;
 }
@@ -180,23 +182,32 @@ static int __must_check queue_invalidate_wait(struct vtd_iommu *iommu,
     /* Now we don't support interrupt method */
     if ( sw )
     {
-        s_time_t timeout;
-
-        /* In case all wait descriptor writes to same addr with same data */
-        timeout = NOW() + MILLISECS(flush_dev_iotlb ?
-                                    iommu_dev_iotlb_timeout : VTD_QI_TIMEOUT);
+        static unsigned int __read_mostly threshold = 1;
+        s_time_t start = NOW();
+        s_time_t timeout = start + (flush_dev_iotlb
+                                    ? iommu_dev_iotlb_timeout
+                                    : 100) * MILLISECS(threshold);
 
         while ( ACCESS_ONCE(*this_poll_slot) != QINVAL_STAT_DONE )
         {
-            if ( NOW() > timeout )
+            if ( timeout && NOW() > timeout )
             {
-                print_qi_regs(iommu);
+                threshold |= threshold << 1;
                 printk(XENLOG_WARNING VTDPREFIX
-                       " Queue invalidate wait descriptor timed out\n");
-                return -ETIMEDOUT;
+                       " IOMMU#%u: QI%s wait descriptor taking too long\n",
+                       iommu->index, flush_dev_iotlb ? " dev" : "");
+                print_qi_regs(iommu);
+                timeout = 0;
             }
             cpu_relax();
         }
+
+        if ( !timeout )
+            printk(XENLOG_WARNING VTDPREFIX
+                   " IOMMU#%u: QI%s wait descriptor took %lums\n",
+                   iommu->index, flush_dev_iotlb ? " dev" : "",
+                   (NOW() - start) / 10000000);
+
         return 0;
     }