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

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Message-ID: <AADFC41AFE54684AB9EE6CBC0274A5D19CA31E28@SHSMSX104.ccr.corp.intel.com>
Date:   Wed, 15 May 2019 14:47:38 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "Alex Williamson" <alex.williamson@...hat.com>,
        Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: RE: [PATCH v3 02/16] iommu: Introduce cache_invalidate API

> From: Jean-Philippe Brucker
> Sent: Wednesday, May 15, 2019 7:04 PM
> 
> 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.
> 

I didn't get how ATS works in such case, if device ATC PASID is different
from IOTLB ASID. Who will be responsible for translation in-between?

Thanks
Kevin