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

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Date:   Tue, 14 May 2019 10:44:01 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Jean-Philippe Brucker <jean-philippe.brucker@....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>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v3 02/16] iommu: Introduce cache_invalidate API

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.

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

> Thanks,
> Jean
> 
> > 
> > At the moment the ASID is allocated per iommu domain. With aux
> > domains we should have one ASID per aux domain, Jean-Philippe said.
> > 
> > ASID tags IOTLB S1 entries. As the ASID is part of the "context
> > descriptor" which is owned by the guest, the API must pass it
> > somehow.
> > 
> > 4.4.1.2 CMD_TLBI_NH_ASID(VMID, ASID) invalidation command allows to
> > invalidate all IOTLB S1 entries for a given VMID/ASID and this is
> > the functionality which is currently missing in the API. This is
> > not an address based invalidation or a "pure" PASID based
> > invalidation. At the moment we don't support PASIDs on ARM and I
> > need this capability.
> > 
Got it.
> > Thanks
> > 
> > Eric
> > 
> > 
> >   
> >>> Thanks,
> >>> Jean
> >>>  
> >>>>
> >>>> Domain invalidation would invalidate all the contexts belonging
> >>>> to that domain.
> >>>>
> >>>> Thanks
> >>>>
> >>>> Eric    
> >>
> >> [Jacob Pan]
> >>  
> 

[Jacob Pan]