linux-kernel - Re: [PATCH 3/3] virtio-pmem: Add virtio pmem driver

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Date:   Thu, 27 Sep 2018 09:06:40 -0400 (EDT)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        Qemu Developers <qemu-devel@...gnu.org>,
        linux-nvdimm <linux-nvdimm@...1.01.org>, Jan Kara <jack@...e.cz>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Nitesh Narayan Lal <nilal@...hat.com>,
        Kevin Wolf <kwolf@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Ross Zwisler <ross.zwisler@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        niteshnarayanlal@...mail.com, lcapitulino@...hat.com,
        Igor Mammedov <imammedo@...hat.com>,
        Eric Blake <eblake@...hat.com>
Subject: Re: [PATCH 3/3] virtio-pmem: Add virtio pmem driver


Hello Dan,

> > > + /* The request submission function */
> > > +static int virtio_pmem_flush(struct nd_region *nd_region)
> > > +{
> > > +       int err;

[...]

> > > +       init_waitqueue_head(&req->host_acked);
> > > +       init_waitqueue_head(&req->wq_buf);
> > > +
> > > +       spin_lock_irqsave(&vpmem->pmem_lock, flags);
> > > +       sg_init_one(&sg, req->name, strlen(req->name));
> > > +       sgs[0] = &sg;
> > > +       sg_init_one(&ret, &req->ret, sizeof(req->ret));
> > > +       sgs[1] = &ret;

[...]
> > > +       spin_unlock_irqrestore(&vpmem->pmem_lock, flags);
> > > +       /* When host has read buffer, this completes via host_ack */
> > > +       wait_event(req->host_acked, req->done);
> > 
> > Hmm, this seems awkward if this is called from pmem_make_request. If
> > we need to wait for completion that should be managed by the guest
> > block layer. I.e. make_request should just queue request and then
> > trigger bio_endio() when the response comes back.
> 
> We are plugging VIRTIO based flush callback for virtio_pmem driver. If pmem
> driver (pmem_make_request) has to queue request we have to plug "blk_mq_ops"
> callbacks for corresponding  VIRTIO vqs. AFAICU there is no existing
> multiqueue
> code merged for pmem driver yet, though i could see patches by Dave upstream.
> 

I thought about this and with current infrastructure "make_request" releases spinlock 
and makes current thread/task. All Other threads are free to call 'make_request'/flush 
and similarly wait by releasing the lock. This actually works like a queue of threads 
waiting for notifications from host.

Current pmem code do not have multiqueue support and I am not sure if core pmem code 
needs it. Adding multiqueue support just for virtio-pmem and not for pmem in same driver
will be confusing or require alot of tweaking.

Could you please give your suggestions on this. 
 
Thanks,
Pankaj