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Date:   Thu, 26 Apr 2018 12:43:24 -0400 (EDT)
From:   Pankaj Gupta <pagupta@redhat.com>
To:     Stefan Hajnoczi <stefanha@gmail.com>
Cc:     jack@suse.cz, kvm@vger.kernel.org, david@redhat.com,
        linux-nvdimm@ml01.01.org, ross zwisler <ross.zwisler@intel.com>,
        qemu-devel@nongnu.org, lcapitulino@redhat.com, linux-mm@kvack.org,
        niteshnarayanlal@hotmail.com, mst@redhat.com, hch@infradead.org,
        marcel@redhat.com, nilal@redhat.com,
        haozhong zhang <haozhong.zhang@intel.com>, riel@surriel.com,
        stefanha@redhat.com, pbonzini@redhat.com,
        dan j williams <dan.j.williams@intel.com>, kwolf@redhat.com,
        xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
        linux-kernel@vger.kernel.org, imammedo@redhat.com
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References: <20180425112415.12327-1-pagupta@redhat.com> <20180425112415.12327-4-pagupta@redhat.com> <20180426132406.GC30991@stefanha-x1.localdomain>
Subject: Re: [Qemu-devel] [RFC v2] qemu: Add virtio pmem device
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Thread-Topic: qemu: Add virtio pmem device
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> > +static void virtio_pmem_flush(VirtIODevice *vdev, VirtQueue *vq)
> > +{
> > +    VirtQueueElement *elem;
> > +    VirtIOPMEM *pmem = VIRTIO_PMEM(vdev);
> > +    HostMemoryBackend *backend = MEMORY_BACKEND(pmem->memdev);
> > +    int fd = memory_region_get_fd(&backend->mr);
> > +
> > +    elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> > +    if (!elem) {
> > +        return;
> > +    }
> > +    /* flush raw backing image */
> > +    fsync(fd);
> 
> fsync(2) is a blocking syscall.  This can hang QEMU for an unbounded
> amount of time.

o.k. Main thread will block, agree.

> 
> Please do the fsync from a thread pool.  See block/file-posix.c's
> aio_worker() for an example.

Sure!

> 
> > +static void virtio_pmem_get_config(VirtIODevice *vdev, uint8_t *config)
> > +{
> > +    VirtIOPMEM *pmem = VIRTIO_PMEM(vdev);
> > +    struct virtio_pmem_config *pmemcfg = (struct virtio_pmem_config *)
> > config;
> > +
> > +    pmemcfg->start = pmem->start;
> > +    pmemcfg->size  = pmem->size;
> 
> Endianness.  Please use virtio_st*_p() instead.

sure.

> 
> > +#define VIRTIO_PMEM_PLUG 0
> 
> What is this?

will remove

Thanks,
Pankaj