linux-kernel - Re: [Qemu-devel] [RFC v3] qemu: Add virtio pmem device

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Date:   Fri, 20 Jul 2018 09:04:15 -0400 (EDT)
From:   Pankaj Gupta <pagupta@...hat.com>
To:     Luiz Capitulino <lcapitulino@...hat.com>
Cc:     Stefan Hajnoczi <stefanha@...hat.com>, kwolf@...hat.com,
        haozhong zhang <haozhong.zhang@...el.com>, jack@...e.cz,
        xiaoguangrong eric <xiaoguangrong.eric@...il.com>,
        kvm@...r.kernel.org, riel@...riel.com, linux-nvdimm@...1.01.org,
        david@...hat.com, ross zwisler <ross.zwisler@...el.com>,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        hch@...radead.org, imammedo@...hat.com, mst@...hat.com,
        niteshnarayanlal@...mail.com, pbonzini@...hat.com,
        dan j williams <dan.j.williams@...el.com>, nilal@...hat.com
Subject: Re: [Qemu-devel] [RFC v3] qemu: Add virtio pmem device


> > > > > +
> > > > > +typedef struct VirtIOPMEMresp {
> > > > > +    int ret;
> > > > > +} VirtIOPMEMResp;
> > > > > +
> > > > > +typedef struct VirtIODeviceRequest {
> > > > > +    VirtQueueElement elem;
> > > > > +    int fd;
> > > > > +    VirtIOPMEM *pmem;
> > > > > +    VirtIOPMEMResp resp;
> > > > > +} VirtIODeviceRequest;
> > > > > +
> > > > > +static int worker_cb(void *opaque)
> > > > > +{
> > > > > +    VirtIODeviceRequest *req = opaque;
> > > > > +    int err = 0;
> > > > > +
> > > > > +    /* flush raw backing image */
> > > > > +    err = fsync(req->fd);
> > > > > +    if (err != 0) {
> > > > > +        err = errno;
> > > > > +    }
> > > > > +    req->resp.ret = err;
> > > > 
> > > > Host question: are you returning the guest errno code to the host?
> > > 
> > > No. I am returning error code from the host in-case of host fsync
> > > failure, otherwise returning zero.
> > 
> > I think that's what Luiz meant.  errno constants are not portable
> > between operating systems and architectures.  Therefore they cannot be
> > used in external interfaces in software that expects to communicate with
> > other systems.
> 
> Oh, thanks. Only saw this email now.
> 
> > It will be necessary to define specific constants for virtio-pmem
> > instead of passing errno from the host to guest.
> 
> Yes, defining your own constants work. But I think the only fsync()
> error that will make sense for the guest is EIO. The other errors
> only make sense for the host.

Agree.

Thanks,
Pankaj