linux-kernel - Re: [PATCH v3 02/16] iommu: Introduce cache

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Date:   Wed, 15 May 2019 12:03:45 +0100
From:   Jean-Philippe Brucker <jean-philippe.brucker@....com>
To:     Jacob Pan <jacob.jun.pan@...ux.intel.com>
Cc:     Auger Eric <eric.auger@...hat.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Raj Ashok <ashok.raj@...el.com>,
        Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>
Subject: Re: [PATCH v3 02/16] iommu: Introduce cache_invalidate API

On 14/05/2019 18:44, Jacob Pan wrote:
> Hi Thank you both for the explanation.
> 
> On Tue, 14 May 2019 11:41:24 +0100
> Jean-Philippe Brucker <jean-philippe.brucker@....com> wrote:
> 
>> On 14/05/2019 08:36, Auger Eric wrote:
>>> Hi Jacob,
>>>
>>> On 5/14/19 12:16 AM, Jacob Pan wrote:  
>>>> On Mon, 13 May 2019 18:09:48 +0100
>>>> Jean-Philippe Brucker <jean-philippe.brucker@....com> wrote:
>>>>  
>>>>> On 13/05/2019 17:50, Auger Eric wrote:  
>>>>>>> struct iommu_inv_pasid_info {
>>>>>>> #define IOMMU_INV_PASID_FLAGS_PASID	(1 << 0)
>>>>>>> #define IOMMU_INV_PASID_FLAGS_ARCHID	(1 << 1)
>>>>>>> 	__u32	flags;
>>>>>>> 	__u32	archid;
>>>>>>> 	__u64	pasid;
>>>>>>> };    
>>>>>> I agree it does the job now. However it looks a bit strange to
>>>>>> do a PASID based invalidation in my case - SMMUv3 nested stage -
>>>>>> where I don't have any PASID involved.
>>>>>>
>>>>>> Couldn't we call it context based invalidation then? A context
>>>>>> can be tagged by a PASID or/and an ARCHID.    
>>>>>
>>>>> I think calling it "context" would be confusing as well (I
>>>>> shouldn't have used it earlier), since VT-d uses that name for
>>>>> device table entries (=STE on Arm SMMU). Maybe "addr_space"?
>>>>>  
>>>> I am still struggling to understand what ARCHID is after scanning
>>>> through SMMUv3.1 spec. It seems to be a constant for a given SMMU.
>>>> Why do you need to pass it down every time? Could you point to me
>>>> the document or explain a little more on ARCHID use cases.
>>>> We have three fileds called pasid under this struct
>>>> iommu_cache_invalidate_info{}
>>>> Gets confusing :)  
>>> archid is a generic term. That's why you did not find it in the
>>> spec ;-)
>>>
>>> On ARM SMMU the archid is called the ASID (Address Space ID, up to
>>> 16 bits. The ASID is stored in the Context Descriptor Entry (your
>>> PASID entry) and thus characterizes a given stage 1 translation
>>> "context"/"adress space".  
>>
>> Yes, another way to look at it is, for a given address space:
>> * PASID tags device-IOTLB (ATC) entries.
>> * ASID (here called archid) tags IOTLB entries.
>>
>> They could have the same value, but it depends on the guest's
>> allocation policy which isn't in our control. With my PASID patches
>> for SMMUv3, they have different values. So we need both fields if we
>> intend to invalidate both ATC and IOTLB with a single call.
>>
> For ASID invalidation, there is also page/address selective within an
> ASID, right? I guess it is CMD_TLBI_NH_VA?
> So the single call to invalidate both ATC & IOTLB should share the same
> address information. i.e.
> struct iommu_inv_addr_info {}
> 
> Just out of curiosity, what is the advantage of having guest tag its
> ATC with its own PASID? I thought you were planning to use custom
> ioasid allocator to get PASID from host.

Hm, for the moment I mostly considered the custom ioasid allocator for
Intel platforms. On Arm platforms the SR-IOV model where each VM has its
own PASID space is still very much on the table. This would be the only
model supported by a vSMMU emulation for example, since the SMMU doesn't
have PASID allocation commands.

> Also ASID is 16 bit as Eric said and PASID (substreamID?) is 20 bit,
> right?

Yes. Some implementations have 8-bit ASIDs, but I think those would be
on embedded rather than server class platforms. And yes, if it wasn't
confusing enough, the Arm SMMU uses "SubstreamID" (SSID) for PASIDs :)

Thanks,
Jean