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

[Lu Baolu]

Hi John,

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

Yes. You are right.

> > 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