Hi John,

On 6/17/21 4:00 PM, John Garry wrote:

On 17/06/2021 08:32, Lu Baolu wrote:

quoted

On 6/16/21 7:03 PM, John Garry wrote:

quoted

@@ -4382,9 +4380,9 @@ int __init intel_iommu_init(void)

           * is likely to be much lower than the overhead of 
synchronizing
           * the virtual and physical IOMMU page-tables.
           */
-        if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
-            pr_warn("IOMMU batching is disabled due to 
virtualization");
-            intel_iommu_strict = 1;
+        if (cap_caching_mode(iommu->cap)) {
+            pr_warn("IOMMU batching disallowed due to 
virtualization\n");
+            iommu_set_dma_strict(true);

With this change, VM guest will always show this warning.

Would they have got it before also normally?

I mean, default is intel_iommu_strict=0, so if 
cap_caching_mode(iommu->cap) is true and intel_iommu_strict not set to 1 
elsewhere previously, then we would get this print.

Yes. You are right.

quoted

How about
removing this message? Users could get the same information through the
kernel message added by "[PATCH v13 2/6] iommu: Print strict or lazy
mode at init time".

I think that the print from 2/6 should occur before this print.

Regardless I would think that you would still like to be notified of 
this change in policy, right?

However I now realize that the print is in a loop per iommu, so we would 
get it per iommu:

for_each_active_iommu(iommu, drhd) {
     /*
      * The flush queue implementation does not perform
      * page-selective invalidations that are required for efficient
      * TLB flushes in virtual environments.  The benefit of batching
      * is likely to be much lower than the overhead of synchronizing
      * the virtual and physical IOMMU page-tables.
      */
     if (!intel_iommu_strict && cap_caching_mode(iommu->cap)) {
         pr_warn("IOMMU batching is disabled due to virtualization");
         intel_iommu_strict = 1;
     }
     ...
}

I need to change that. How about this:

bool print_warning = false;

for_each_active_iommu(iommu, drhd) {
     /*
      * The flush queue implementation does not perform
      * page-selective invalidations that are required for efficient
      * TLB flushes in virtual environments.  The benefit of batching
      * is likely to be much lower than the overhead of synchronizing
      * the virtual and physical IOMMU page-tables.
      */
     if (!print_warning && cap_caching_mode(iommu->cap)) {
         pr_warn("IOMMU batching disallowed due to virtualization\n");
         iommu_set_dma_strict(true);
         print_warning = true;
     }
     ...
}

or use pr_warn_once().

 From my p.o.v, pr_xxxx_once() is better.

How about using a pr_info_once()? I don't think it's a warning, it's
just a policy choice in VM environment.

Best regards,
baolu