Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:5 as permitted sender) client-ip=2620:137:e000::3:5;
Date:   Wed, 27 Sep 2023 12:52:31 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     "Tian, Kevin" <kevin.tian@intel.com>
Cc:     "Liu, Yi L" <yi.l.liu@intel.com>,
        "jgg@nvidia.com" <jgg@nvidia.com>,
        "robin.murphy@arm.com" <robin.murphy@arm.com>,
        "baolu.lu@linux.intel.com" <baolu.lu@linux.intel.com>,
        "joro@8bytes.org" <joro@8bytes.org>,
        "cohuck@redhat.com" <cohuck@redhat.com>,
        "eric.auger@redhat.com" <eric.auger@redhat.com>,
        "nicolinc@nvidia.com" <nicolinc@nvidia.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "mjrosato@linux.ibm.com" <mjrosato@linux.ibm.com>,
        "chao.p.peng@linux.intel.com" <chao.p.peng@linux.intel.com>,
        "yi.y.sun@linux.intel.com" <yi.y.sun@linux.intel.com>,
        "peterx@redhat.com" <peterx@redhat.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>,
        "shameerali.kolothum.thodi@huawei.com" 
        <shameerali.kolothum.thodi@huawei.com>,
        "lulu@redhat.com" <lulu@redhat.com>,
        "suravee.suthikulpanit@amd.com" <suravee.suthikulpanit@amd.com>,
        "iommu@lists.linux.dev" <iommu@lists.linux.dev>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@intel.com>,
        "Martins, Joao" <joao.m.martins@oracle.com>
Subject: Re: [RFC 3/3] vfio/pci: Expose PCIe PASID capability to userspace
Message-ID: <20230927125231.3aacde62.alex.williamson@redhat.com>
In-Reply-To: <BN9PR11MB5276375C4BE33AC0632321A68CC2A@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20230926093121.18676-1-yi.l.liu@intel.com>
        <20230926093121.18676-4-yi.l.liu@intel.com>
        <BN9PR11MB5276375C4BE33AC0632321A68CC2A@BN9PR11MB5276.namprd11.prod.outlook.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

On Wed, 27 Sep 2023 08:07:54 +0000
"Tian, Kevin" <kevin.tian@intel.com> wrote:

> > From: Liu, Yi L <yi.l.liu@intel.com>
> > Sent: Tuesday, September 26, 2023 5:31 PM
> > 
> > This exposes PCIe PASID capability to userspace and where to emulate this
> > capability if wants to further expose it to VM.
> > 
> > And this only exposes PASID capability for devices which has PCIe PASID
> > extended struture in its configuration space. While for VFs, userspace
> > is still unable to see this capability as SR-IOV spec forbides VF to
> > implement PASID capability extended structure. It is a TODO in future.
> > Related discussion can be found in below links:
> > 
> > https://lore.kernel.org/kvm/20200407095801.648b1371@w520.home/
> > https://lore.kernel.org/kvm/BL1PR11MB5271A60035EF591A5BE8AC878C01A
> > @BL1PR11MB5271.namprd11.prod.outlook.com/
> >   
> 
> Yes, we need a decision for VF case.
> 
> If the consensus is to continue exposing the PASID capability in vfio-pci
> config space by developing a kernel quirk mechanism to find offset for
> VF, then this patch for PF is orthogonal to that VF work and can go as it is.
> 
> But if the decision is to have a device feature for the user to enumerate
> the vPASID capability and let the VMM take care of finding the vPASID
> cap offset, then better we start doing that for PF too since it's not good
> to have two enumeration interfaces for PF/VF respectively.

Note also that QEMU implements a lazy algorithm for exposing
capabilities, the default is to expose them, so we need to consider
existing VMs seeing a new read-only PASID capability on an assigned PF.

That might support an alternate means to expose the capability.

> My preference is via device feature given Qemu already includes lots of
> quirks for vfio-pci devices. Another reason is that when supporting vPASID
> with SIOV there are some arch constraints which the driver needs to
> report to the user to follow (e.g.  don't assign ENQCMD-capable sibling
> vdev's to a same guest, etc.).

?!

> A device feature interface can better
> encapsulate everything related to vPASID in one place.

Sorry if I don't remember, have you posted a proposal for the device
feature interface?  Thanks,

Alex