linux-kernel - Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE

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Message-ID: <CACycT3uCSLUDVpQHdrmuxSuoBDg-4n22t+N-Jm2GoNNp9JYB2w@mail.gmail.com>
Date:   Thu, 24 Jun 2021 12:46:30 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian.brauner@...onical.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        Mika Penttilä <mika.penttila@...tfour.com>,
        Dan Carpenter <dan.carpenter@...cle.com>, joro@...tes.org,
        Greg KH <gregkh@...uxfoundation.org>, songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev@...r.kernel.org, kvm <kvm@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

On Thu, Jun 24, 2021 at 11:35 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2021/6/23 下午1:50, Yongji Xie 写道:
> > On Wed, Jun 23, 2021 at 11:31 AM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> 在 2021/6/22 下午4:14, Yongji Xie 写道:
> >>> On Tue, Jun 22, 2021 at 3:50 PM Jason Wang <jasowang@...hat.com> wrote:
> >>>> 在 2021/6/22 下午3:22, Yongji Xie 写道:
> >>>>>> We need fix a way to propagate the error to the userspace.
> >>>>>>
> >>>>>> E.g if we want to stop the deivce, we will delay the status reset until
> >>>>>> we get respose from the userspace?
> >>>>>>
> >>>>> I didn't get how to delay the status reset. And should it be a DoS
> >>>>> that we want to fix if the userspace doesn't give a response forever?
> >>>> You're right. So let's make set_status() can fail first, then propagate
> >>>> its failure via VHOST_VDPA_SET_STATUS.
> >>>>
> >>> OK. So we only need to propagate the failure in the vhost-vdpa case, right?
> >>
> >> I think not, we need to deal with the reset for virtio as well:
> >>
> >> E.g in register_virtio_devices(), we have:
> >>
> >>           /* We always start by resetting the device, in case a previous
> >>            * driver messed it up.  This also tests that code path a
> >> little. */
> >>         dev->config->reset(dev);
> >>
> >> We probably need to make reset can fail and then fail the
> >> register_virtio_device() as well.
> >>
> > OK, looks like virtio_add_status() and virtio_device_ready()[1] should
> > be also modified if we need to propagate the failure in the
> > virtio-vdpa case. Or do we only need to care about the reset case?
> >
> > [1] https://lore.kernel.org/lkml/20210517093428.670-1-xieyongji@bytedance.com/
>
>
> My understanding is DRIVER_OK is not something that needs to be validated:
>
> "
>
> DRIVER_OK (4)
> Indicates that the driver is set up and ready to drive the device.
>
> "
>
> Since the spec doesn't require to re-read the and check if DRIVER_OK is
> set in 3.1.1 Driver Requirements: Device Initialization.
>
> It's more about "telling the device that driver is ready."
>
> But we don have some status bit that requires the synchronization with
> the device.
>
> 1) FEATURES_OK, spec requires to re-read the status bit to check whether
> or it it was set by the device:
>
> "
>
> Re-read device status to ensure the FEATURES_OK bit is still set:
> otherwise, the device does not support our subset of features and the
> device is unusable.
>
> "
>
> This is useful for some device which can only support a subset of the
> features. E.g a device that can only work for packed virtqueue. This
> means the current design of set_features won't work, we need either:
>
> 1a) relay the set_features request to userspace
>
> or
>
> 1b) introduce a mandated_device_features during device creation and
> validate the driver features during the set_features(), and don't set
> FEATURES_OK if they don't match.
>
>
> 2) Some transports (PCI) requires to re-read the status to ensure the
> synchronization.
>
> "
>
> After writing 0 to device_status, the driver MUST wait for a read of
> device_status to return 0 before reinitializing the device.
>
> "
>
> So we need to deal with both FEATURES_OK and reset, but probably not
> DRIVER_OK.
>

OK, I see. Thanks for the explanation. One more question is how about
clearing the corresponding status bit in get_status() rather than
making set_status() fail. Since the spec recommends this way for
validation which is done in virtio_dev_remove() and
virtio_finalize_features().

Thanks,
Yongji