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from redhat.com ([2a10:800a:cdef:0:114d:2085:61e4:7b41]) by smtp.gmail.com with ESMTPSA id x189sm1442422wmg.17.2021.04.21.01.03.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 21 Apr 2021 01:03:27 -0700 (PDT) Date: Wed, 21 Apr 2021 04:03:24 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, parav@nvidia.com, elic@nvidia.com, "Zhu, Lingshan" Subject: Re: [RFC PATCH] vdpa: mandate 1.0 device Message-ID: <20210421035331-mutt-send-email-mst@kernel.org> References: <20210408082648.20145-1-jasowang@redhat.com> <20210408115834-mutt-send-email-mst@kernel.org> <20210409115343-mutt-send-email-mst@kernel.org> <42891807-cb24-5352-f8cb-798e9d1a1854@redhat.com> <20210412050730-mutt-send-email-mst@kernel.org> <01918e14-7f7a-abf2-5864-292a32f0233c@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Apr 21, 2021 at 03:41:36PM +0800, Jason Wang wrote: > > 在 2021/4/12 下午5:23, Jason Wang 写道: > > > > 在 2021/4/12 下午5:09, Michael S. Tsirkin 写道: > > > On Mon, Apr 12, 2021 at 02:35:07PM +0800, Jason Wang wrote: > > > > 在 2021/4/10 上午12:04, Michael S. Tsirkin 写道: > > > > > On Fri, Apr 09, 2021 at 12:47:55PM +0800, Jason Wang wrote: > > > > > > 在 2021/4/8 下午11:59, Michael S. Tsirkin 写道: > > > > > > > On Thu, Apr 08, 2021 at 04:26:48PM +0800, Jason Wang wrote: > > > > > > > > This patch mandates 1.0 for vDPA devices. The goal is to have the > > > > > > > > semantic of normative statement in the virtio > > > > > > > > spec and eliminate the > > > > > > > > burden of transitional device for both vDPA bus and vDPA parent. > > > > > > > > > > > > > > > > uAPI seems fine since all the vDPA parent mandates > > > > > > > > VIRTIO_F_ACCESS_PLATFORM which implies 1.0 devices. > > > > > > > > > > > > > > > > For legacy guests, it can still work since Qemu will mediate when > > > > > > > > necessary (e.g doing the endian conversion). > > > > > > > > > > > > > > > > Signed-off-by: Jason Wang > > > > > > > Hmm. If we do this, don't we still have a problem with > > > > > > > legacy drivers which don't ack 1.0? > > > > > > Yes, but it's not something that is introduced in this > > > > > > commit. The legacy > > > > > > driver never work ... > > > > > My point is this neither fixes or prevents this. > > > > > > > > > > So my suggestion is to finally add ioctls along the lines > > > > > of PROTOCOL_FEATURES of vhost-user. > > > > > > > > > > Then that one can have bits for legacy le, legacy be and modern. > > > > > > > > > > BTW I looked at vhost-user and it does not look like that > > > > > has a solution for this problem either, right? > > > > > > > > Right. > > > > > > > > > > > > > > > > > > > > Note 1.0 affects ring endianness which is not mediated in QEMU > > > > > > > so QEMU can't pretend to device guest is 1.0. > > > > > > Right, I plan to send patches to do mediation in the > > > > > > Qemu to unbreak legacy > > > > > > drivers. > > > > > > > > > > > > Thanks > > > > > I frankly think we'll need PROTOCOL_FEATURES anyway, it's > > > > > too useful ... > > > > > so why not teach drivers about it and be done with it? You > > > > > can't emulate > > > > > legacy on modern in a cross endian situation because of vring > > > > > endian-ness ... > > > > > > > > So the problem still. This can only work when the hardware can support > > > > legacy vring endian-ness. > > > > > > > > Consider: > > > > > > > > 1) the leagcy driver support is non-normative in the spec > > > > 2) support a transitional device in the kenrel may requires the > > > > hardware > > > > support and a burden of kernel codes > > > > > > > > I'd rather simply drop the legacy driver support > > > > > > My point is this patch does not drop legacy support. It merely mandates > > > modern support. > > > > > > I am not sure I get here. This patch fails the set_feature if VERSION_1 > > is not negotiated. This means: > > > > 1) vDPA presents a modern device instead of transitonal device > > 2) legacy driver can't be probed > > > > What I'm missing? > > > Hi Michael: > > Do you agree to find the way to present modern device? We need a conclusion > to make the netlink API work to move forward. > > Thanks I think we need a way to support legacy with no data path overhead. qemu setting VERSION_1 for a legacy guest affects the ring format so it does not really work. This seems to rule out emulating config space entirely in userspace. So I think we should add an ioctl along the lines of protocol features. Then I think we can reserve feature bits for config space format: legacy LE, legacy BE, modern. Querying the feature bits will provide us with info about what does the device support. Acking them will tell device what does guest need. > > > > > > > > > > > > to have a simple and easy > > > > abstarction in the kenrel. For legacy driver in the guest, > > > > hypervisor is in > > > > charge of the mediation: > > > > > > > > 1) config space access endian conversion > > > > 2) using shadow virtqueue to change the endian in the vring > > > > > > > > Thanks > > > I'd like to avoid shadow virtqueue hacks if at all possible. > > > Last I checked performance wasn't much better than just emulating > > > virtio in software. > > > > > > I think the legacy driver support is just a nice to have. Or do you see > > any value to that? I guess for mellanox and intel, only modern device is > > supported in the hardware. > > > > Thanks > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > --- > > > > > > > >     include/linux/vdpa.h | 6 ++++++ > > > > > > > >     1 file changed, 6 insertions(+) > > > > > > > > > > > > > > > > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h > > > > > > > > index 0fefeb976877..cfde4ec999b4 100644 > > > > > > > > --- a/include/linux/vdpa.h > > > > > > > > +++ b/include/linux/vdpa.h > > > > > > > > @@ -6,6 +6,7 @@ > > > > > > > >     #include > > > > > > > >     #include > > > > > > > >     #include > > > > > > > > +#include > > > > > > > >     /** > > > > > > > >      * vDPA callback definition. > > > > > > > > @@ -317,6 +318,11 @@ static inline int > > > > > > > > vdpa_set_features(struct vdpa_device *vdev, u64 > > > > > > > > features) > > > > > > > >     { > > > > > > > >             const struct vdpa_config_ops *ops = vdev->config; > > > > > > > > +        /* Mandating 1.0 to have semantics of > > > > > > > > normative statements in > > > > > > > > +         * the spec. */ > > > > > > > > +        if (!(features & BIT_ULL(VIRTIO_F_VERSION_1))) > > > > > > > > +        return -EINVAL; > > > > > > > > + > > > > > > > >         vdev->features_valid = true; > > > > > > > >             return ops->set_features(vdev, features); > > > > > > > >     } > > > > > > > > -- > > > > > > > > 2.25.1 > >