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From:   "Rustad, Mark D" <mark.d.rustad@intel.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
CC:     Alexander Duyck <alexander.duyck@gmail.com>,
        "Daly, Dan" <dan.daly@intel.com>,
        Bjorn Helgaas <bhelgaas@google.com>,
        "Duyck, Alexander H" <alexander.h.duyck@intel.com>,
        "linux-pci@vger.kernel.org" <linux-pci@vger.kernel.org>,
        "virtio-dev@lists.oasis-open.org" <virtio-dev@lists.oasis-open.org>,
        kvm-devel <kvm@vger.kernel.org>, Netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        "linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
        "Busch, Keith" <keith.busch@intel.com>,
        "netanel@amazon.com" <netanel@amazon.com>,
        Don Dutile <ddutile@redhat.com>,
        Maximilian Heyne <mheyne@amazon.de>,
        "Wang, Liang-min" <liang-min.wang@intel.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Christoph Hellwig <hch@lst.de>,
        "dwmw@amazon.co.uk" <dwmw@amazon.co.uk>
Subject: Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for
 unmanaged SR-IOV on virtio_pci devices
Thread-Topic: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for
 unmanaged SR-IOV on virtio_pci devices
Thread-Index: AQHTvI1siq32SjdMxU+NwObgk2U5/6PThamAgAABlwCAHA/fgIAASHYAgAAPkYCAAA4nAA==
Date:   Tue, 3 Apr 2018 19:18:23 +0000
Message-ID: <0927465E-0B56-4903-9F5E-288A7D9C46E6@intel.com>
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On Apr 3, 2018, at 11:27 AM, Michael S. Tsirkin <mst@redhat.com> wrote:

>> I'm not sure why you would need a feature bit. The capability is
>> controlled via PCI configuration space. If it is present the device
>> has the capability. If it is not then it does not.
>>
>> Basically if the PCI configuration space is not present then the sysfs
>> entries will not be spawned and nothing will attempt to use this
>> function.
>>
>> - ALex
>
> It's about compability with older guests which ignore the
> capability.
>
> The feature is thus helpful so host knows whether guest supports VFs.

This is not about a guest creating its own VFs. This is about a host PF  
that happens to have a virtio interface to be able to create virtio VFs  
that can be assigned to guests. Nothing changes at all from a guest  
perspective. Or maybe I am not understanding what you mean by "whether  
guest supports VFs".

--
Mark Rustad, Networking Division, Intel Corporation

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