Re: [PATCH] iommu/arm-smmu-v3: Enable PCI ATS in passthrough mode as well

[Ganapatrao Kulkarni]

On 03-02-2023 05:30 pm, Jean-Philippe Brucker wrote:
> On Thu, Feb 02, 2023 at 04:40:53AM -0800, Ganapatrao Kulkarni wrote:
> > The current smmu-v3 driver does not enable PCI ATS for physical functions
> > of ATS capable End Points when booted in smmu bypass mode
> > (iommu.passthrough=1). This will not allow virtual functions to enable
> > ATS(even though EP supports it) while they are attached to a VM using
> > VFIO driver.
> >
> > This patch adds changes to enable ATS support for physical functions
> > in passthrough/bypass mode as well.
> [...]
> > @@ -2453,8 +2458,7 @@ static int arm_smmu_attach_dev(struct iommu_domain *domain, struct device *dev)
> >   
> >   	master->domain = smmu_domain;
> >   
> > -	if (smmu_domain->stage != ARM_SMMU_DOMAIN_BYPASS)
> > -		master->ats_enabled = arm_smmu_ats_supported(master);
> > +	master->ats_enabled = arm_smmu_ats_supported(master);
>
> I should have added a comment for this. Only found the reason in an old
> cover letter [1]:
>
> "When no translation stages are enabled (0b100), ATS Translation Requests
> (and Translated traffic, if SMMU_CR0.ATSCHK == 1) are denied as though
> EATS == 0b00; the actual value of the EATS field is IGNORED. Such a
> Translation Request causes F_BAD_ATS_TREQ and Translated traffic causes
> F_TRANSL_FORBIDDEN."
>
> (See 3.9.1.1 "Responses to ATS Translation Requests and Translated
> transactions" and 5.2 "Stream Table Entry")
>
> So I don't think we can enable ATS for bypass domains :/ The PF needs to
> be in translated mode in that case.

Are you intending to say smmu-v3 driver/spec will not support ATS to a 
VF, if it's PF is in bypass?

> I can send a patch adding the comment next cycle.

I am more keen to know, how we enable ATS to a VF of ATS capable EP when 
it's PF is in bypass?
or it is mandatory to have a PF also translated? then that should be 
captured somewhere in documentation.


> Thanks,
> Jean
>
> [1] https://lore.kernel.org/linux-iommu/20190409165245.26500-1-jean-philippe.brucker@xxxxxxx/

Thanks,
Ganapat