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

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Date:   Fri, 3 Feb 2023 12:05:11 +0000
From:   Will Deacon <will@...nel.org>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, robin.murphy@....com, joro@...tes.org,
        darren@...amperecomputing.com, scott@...amperecomputing.com
Subject: Re: [PATCH] iommu/arm-smmu-v3: Enable PCI ATS in passthrough mode as
 well

On Fri, Feb 03, 2023 at 12:00:16PM +0000, 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.
> 
> I can send a patch adding the comment next cycle.

Yes, please!

Will