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

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Message-ID: <Y9z3UG+wEBn4i3G2@myrica>
Date:   Fri, 3 Feb 2023 12:00:16 +0000
From:   Jean-Philippe Brucker <jean-philippe@...aro.org>
To:     Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, robin.murphy@....com, will@...nel.org,
        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 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.

Thanks,
Jean

[1] https://lore.kernel.org/linux-iommu/20190409165245.26500-1-jean-philippe.brucker@arm.com/