linux-kernel - Re: [PATCH v2 09/19] iommu/vt-d: Enlightened PASID allocation

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Message-ID: <20190425160156.4f5decaa@jacob-builder>
Date:   Thu, 25 Apr 2019 16:01:56 -0700
From:   Jacob Pan <jacob.jun.pan@...ux.intel.com>
To:     Auger Eric <eric.auger@...hat.com>
Cc:     "Liu, Yi L" <yi.l.liu@...el.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>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Christoph Hellwig <hch@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Andriy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        jacob.jun.pan@...ux.intel.com
Subject: Re: [PATCH v2 09/19] iommu/vt-d: Enlightened PASID allocation

On Thu, 25 Apr 2019 09:40:31 +0200
Auger Eric <eric.auger@...hat.com> wrote:

> Hi Liu,
> 
> On 4/25/19 9:12 AM, Liu, Yi L wrote:
> > Hi Eric,
> >   
> >> From: Auger Eric [mailto:eric.auger@...hat.com]
> >> Sent: Thursday, April 25, 2019 1:28 AM
> >> To: Jacob Pan <jacob.jun.pan@...ux.intel.com>;
> >> iommu@...ts.linux-foundation.org; Subject: Re: [PATCH v2 09/19]
> >> iommu/vt-d: Enlightened PASID allocation
> >>
> >> Hi Jacob,
> >>
> >> On 4/24/19 1:31 AM, Jacob Pan wrote:  
> >>> From: Lu Baolu <baolu.lu@...ux.intel.com>
> >>>
> >>> If Intel IOMMU runs in caching mode, a.k.a. virtual IOMMU, the
> >>> IOMMU driver should rely on the emulation software to allocate
> >>> and free PASID IDs.  
> >> Do we make the decision depending on the CM or depending on the
> >> VCCAP_REG?
> >>
> >> VCCAP_REG description says:
> >>
> >> If Set, software must use Virtual Command Register interface to
> >> allocate and free PASIDs.  
> > 
> > The answer is it depends on the ECAP.VCS and then the PASID
> > allocation bit in VCCAP_REG. But VCS bit implies the iommu is a
> > software implementation (vIOMMU) of vt-d architecture. Pls refer to
> > the descriptions of "Virtual Command Support" in vt-d 3.0 spec.
> > 
> > "Hardware implementations of this architecture report a value of 0
> > in this field. Software implementations (emulation) of this
> > architecture may report VCS=1."  
> 
> OK I understand. But strictly speaking a vIOMMU may not implement CM.
> But that's nitpicking ;-)
> 
CAP.CM (caching mode) and ECAP.VCS(virtual command support) are
separate. I think we are mixing the two here since both are sufficient
condition to indicate whether we are running in a guest.
> Thanks
> 
> Eric
> > 
> > Thanks,
> > Yi Liu
> >   

[Jacob Pan]