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From:   Jason Wang <jasowang@redhat.com>
Date:   Thu, 9 Sep 2021 17:31:23 +0800
Message-ID: <CACGkMEvt5G-f4U=ReXNdmBcbywzsAhV1eYL2bP2Wp4RA_pKrYQ@mail.gmail.com>
Subject: Re: [PATCH 0/6] vDPA driver for legacy virtio-pci device
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Wu Zongyong <wuzongyong@linux.alibaba.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        wei.yang1@linux.alibaba.com
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On Thu, Sep 9, 2021 at 5:21 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Thu, Sep 09, 2021 at 11:05:06AM +0800, Jason Wang wrote:
> > On Wed, Sep 8, 2021 at 8:22 PM Wu Zongyong <wuzongyong@linux.alibaba.com> wrote:
> > >
> > > This series implements the vDPA driver for legacy virtio-pci device.
> > > Currently we already have the vDPA driver for modern virtio-pci device
> > > only, but there are some legacy virtio-pci devices conforming to the
> > > virtio-pci specifications of 0.9.x or older versions. For example,
> > > ENI(Elastic Network Interface) of Alibaba ECS baremetal instance is a
> > > hardware virtio network device which follows the Virtio PCI Card 0.9.5
> > > Draft specification. Such legacy virtio-pci devices have some
> > > inconsistent behaviour with modern virtio-pci devices, so some common
> > > codes are split out and modern device specific codes are moved to a
> > > separated file.
> >
> > What worries me a little bit are:
> >
> > 1) vDPA requires IOMMU_PLATFORM to be supported by the device to work,
> > if I understand ENI correctly, it's a legacy device so it can't
> > support ACCESS_PLATFORM. Or is it a legacy device that supports
> > ACCESS_PLATFORM implicitly.
> > 2) vDPA tries to present a 1.0 device, in this case the behavior could
> > be ruled by the spec. If we tries to present an 1.0 device on top of
> > legacy device we may suffer a lot of issues:
> >
> > - endian issue: 1.0 use le but legacy may use native endian
> > - queue_enable semantic which is missed in the legacy
> > - virtqueue size, as you mentioned below
>
> So this all kind of works when guest and host are
> strongly ordered and LE. Case in point x86.
> Question is how do we limit this to an x86 guest?
> Add a new ioctl declaring that this is the case?

I think the most simple way is to disable the driver on non LE host
(assuming it tries to use native endian which is kind of impossible).

Thanks

>
> --
> MST
>