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

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Message-ID: <3a681b82-d840-6ef1-40dd-358f34c8be9c@os.amperecomputing.com>
Date:   Mon, 6 Feb 2023 22:50:00 +0530
From:   Ganapatrao Kulkarni <gankulkarni@...amperecomputing.com>
To:     Jean-Philippe Brucker <jean-philippe@...aro.org>
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 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@arm.com/
> 

Thanks,
Ganapat