Re: [PATCH v14 6/6] iommu: Remove mode argument from iommu_set_dma_strict()

[John Garry]

On 21/06/2021 12:59, Robin Murphy wrote:

+ Nadav

> > On a personal level I would be happy with that approach, but I think
> > it's better to not start changing things right away in a follow-up series.
> >
> > So how about we add this patch (which replaces 6/6 "iommu: Remove mode
> > argument from iommu_set_dma_strict()")?
> >
> > Robin, any opinion?
> For me it boils down to whether there are any realistic workloads where
> non-strict mode*would*  still perform better under virtualisation. The
> only reason for the user to explicitly pass "iommu.strict=0" is because
> they expect it to increase unmap performance; if it's only ever going to
> lead to an unexpected performance loss, I don't see any value in
> overriding the kernel's decision purely for the sake of subservience.
>
> If there*are*  certain valid cases for allowing it for people who really
> know what they're doing, then we should arguably also log a counterpart
> message to say "we're honouring your override but beware it may have the
> opposite effect to what you expect" for the benefit of other users who
> assume it's a generic go-faster knob. At that point it starts getting
> non-trivial enough that I'd want to know for sure it's worthwhile.
>
> The other reason this might be better to revisit later is that an AMD
> equivalent is still in flight[1], and there might be more that can
> eventually be factored out. I think both series are pretty much good to
> merge for 5.14, but time's already tight to sort out the conflicts which
> exist as-is, without making them any worse.

ok, fine. Can revisit.

As for getting these merged, I'll dry-run merging both of those series 
to see the conflicts. It doesn't look too problematic from a glance.

Cheers,
John

> Robin.
>
> [1]
> https://lore.kernel.org/linux-iommu/20210616100500.174507-3-namit@xxxxxxxxxx/
>
> > ------->8---------
> >
> > [PATCH] iommu/vt-d: Make "iommu.strict" override batching due to
> >    virtualization
> >
> > As a change in policy, make iommu.strict cmdline argument override
> > whether we disable batching due to virtualization.
> >
> > The API of iommu_set_dma_strict() is changed to accept a "force"
> > argument, which means that we always set iommu_dma_strict true,
> > regardless of whether we already set via cmdline. Also return a boolean,
> > to tell whether iommu_dma_strict was set or not.
> >
> > Note that in all pre-existing callsites of iommu_set_dma_strict(),
> > argument strict was true, so this argument is dropped.
> >
> > Signed-off-by: John Garry<john.garry@xxxxxxxxxx>
> >
> > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> > index 06666f9d8116..e8d65239b359 100644
> > --- a/drivers/iommu/intel/iommu.c
> > +++ b/drivers/iommu/intel/iommu.c
> > @@ -4380,10 +4380,8 @@ 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 (cap_caching_mode(iommu->cap)) {
> > +        if (cap_caching_mode(iommu->cap) && iommu_set_dma_strict(false))
> >                pr_info_once("IOMMU batching disallowed due to
> > virtualization\n");
> > -            iommu_set_dma_strict(true);
> > -        }
> >            iommu_device_sysfs_add(&iommu->iommu, NULL,
> >                           intel_iommu_groups,
> >                           "%s", iommu->name);
> > diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> > index 60b1ec42e73b..1434bee64af3 100644
> > --- a/drivers/iommu/iommu.c
> > +++ b/drivers/iommu/iommu.c
> > @@ -349,10 +349,14 @@ static int __init iommu_dma_setup(char *str)
> >    }
> >    early_param("iommu.strict", iommu_dma_setup);
> >
> > -void iommu_set_dma_strict(bool strict)
> > +/* Return true if we set iommu_dma_strict */
> > +bool iommu_set_dma_strict(bool force)
> >    {
> > -    if (strict || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT))
> > -        iommu_dma_strict = strict;
> > +    if (force || !(iommu_cmd_line & IOMMU_CMD_LINE_STRICT)) {
> > +        iommu_dma_strict = true;
> > +        return true;
> > +    }
> > +    return false;
> >    }
> >
> >    bool iommu_get_dma_strict(struct iommu_domain *domain)
> > diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> > index 32d448050bf7..f17b20234296 100644
> > --- a/include/linux/iommu.h
> > +++ b/include/linux/iommu.h
> > @@ -476,7 +476,7 @@ int iommu_enable_nesting(struct iommu_domain *domain);
> >    int iommu_set_pgtable_quirks(struct iommu_domain *domain,
> >            unsigned long quirks);
> >
> > -void iommu_set_dma_strict(bool val);
> > +bool iommu_set_dma_strict(bool force);
> >    bool iommu_get_dma_strict(struct iommu_domain *domain);
> >
> >    extern int report_iommu_fault(struct iommu_domain *domain, struct
> > device *dev,