Re: [PATCH v13 4/6] iommu/vt-d: Add support for IOMMU default DMA mode build options

[John Garry <john.garry@xxxxxxxxxx>]

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