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

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Message-ID: <20190514105744.706a5818@jacob-builder>
Date:   Tue, 14 May 2019 10:57:44 -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

On Tue, 14 May 2019 10:44:01 -0700
Jacob Pan <jacob.jun.pan@...ux.intel.com> 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 {}
> 
Nevermind for this question. archid field is already in the addr_info.
Sorry.
> 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]

[Jacob Pan]