netdev - Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

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Message-ID: <20220817063450-mutt-send-email-mst@kernel.org>
Date:   Wed, 17 Aug 2022 06:37:13 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     "Zhu, Lingshan" <lingshan.zhu@...el.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>, jasowang@...hat.com,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, parav@...dia.com, xieyongji@...edance.com,
        gautam.dawar@....com
Subject: Re: [PATCH 2/2] vDPA: conditionally read fields in virtio-net dev

On Wed, Aug 17, 2022 at 05:43:22PM +0800, Zhu, Lingshan wrote:
> 
> 
> On 8/17/2022 5:39 PM, Michael S. Tsirkin wrote:
> > On Wed, Aug 17, 2022 at 05:13:59PM +0800, Zhu, Lingshan wrote:
> > > 
> > > On 8/17/2022 4:55 PM, Michael S. Tsirkin wrote:
> > > > On Wed, Aug 17, 2022 at 10:14:26AM +0800, Zhu, Lingshan wrote:
> > > > > Yes it is a little messy, and we can not check _F_VERSION_1 because of
> > > > > transitional devices, so maybe this is the best we can do for now
> > > > I think vhost generally needs an API to declare config space endian-ness
> > > > to kernel. vdpa can reuse that too then.
> > > Yes, I remember you have mentioned some IOCTL to set the endian-ness,
> > > for vDPA, I think only the vendor driver knows the endian,
> > > so we may need a new function vdpa_ops->get_endian().
> > > In the last thread, we say maybe it's better to add a comment for now.
> > > But if you think we should add a vdpa_ops->get_endian(), I can work
> > > on it for sure!
> > > 
> > > Thanks
> > > Zhu Lingshan
> > I think QEMU has to set endian-ness. No one else knows.
> Yes, for SW based vhost it is true. But for HW vDPA, only
> the device & driver knows the endian, I think we can not
> "set" a hardware's endian.

QEMU knows the guest endian-ness and it knows that
device is accessed through the legacy interface.
It can accordingly send endian-ness to the kernel and
kernel can propagate it to the driver.

> So if you think we should add a vdpa_ops->get_endian(),
> I will drop these comments in the next version of
> series, and work on a new patch for get_endian().
> 
> Thanks,
> Zhu Lingshan

Guests don't get endian-ness from devices so this seems pointless.

-- 
MST