linux-kernel - Re: [RFC v2] qemu: Add virtio pmem device

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Message-ID: <20180426132406.GC30991@stefanha-x1.localdomain>
Date:   Thu, 26 Apr 2018 14:24:06 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-nvdimm@...1.01.org,
        linux-mm@...ck.org, jack@...e.cz, stefanha@...hat.com,
        dan.j.williams@...el.com, riel@...riel.com,
        haozhong.zhang@...el.com, nilal@...hat.com, kwolf@...hat.com,
        pbonzini@...hat.com, ross.zwisler@...el.com, david@...hat.com,
        xiaoguangrong.eric@...il.com, hch@...radead.org, marcel@...hat.com,
        mst@...hat.com, niteshnarayanlal@...mail.com, imammedo@...hat.com,
        lcapitulino@...hat.com
Subject: Re: [RFC v2] qemu: Add virtio pmem device

On Wed, Apr 25, 2018 at 04:54:15PM +0530, Pankaj Gupta wrote:
> +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.

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

> +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.

> +#define VIRTIO_PMEM_PLUG 0

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