linux-kernel - Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for unmanaged SR-IOV on virtio

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Date:   Tue, 3 Apr 2018 19:18:23 +0000
From:   "Rustad, Mark D" <mark.d.rustad@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Alexander Duyck <alexander.duyck@...il.com>,
        "Daly, Dan" <dan.daly@...el.com>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        kvm-devel <kvm@...r.kernel.org>, Netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-nvme@...ts.infradead.org" <linux-nvme@...ts.infradead.org>,
        "Busch, Keith" <keith.busch@...el.com>,
        "netanel@...zon.com" <netanel@...zon.com>,
        Don Dutile <ddutile@...hat.com>,
        Maximilian Heyne <mheyne@...zon.de>,
        "Wang, Liang-min" <liang-min.wang@...el.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Christoph Hellwig <hch@....de>,
        "dwmw@...zon.co.uk" <dwmw@...zon.co.uk>
Subject: Re: [virtio-dev] [pci PATCH v7 2/5] virtio_pci: Add support for
 unmanaged SR-IOV on virtio_pci devices

On Apr 3, 2018, at 11:27 AM, Michael S. Tsirkin <mst@...hat.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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