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SubjectRe: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA mode build options
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On 17/06/2021 08:32, Lu Baolu wrote:
> On 6/16/21 7:03 PM, John Garry wrote:
>> @@ -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.

> 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().

Thanks,
John

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