Re: [Qemu-devel] [PATCH v2 2/4] nvdimm: warn if the backend is not a DAX device

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Jun 6, 2017 at 12:22 AM, Haozhong Zhang
<haozhong.zhang@intel.com> wrote:
> Applications in Linux guest that use device-dax never trigger flush
> that can be trapped by KVM/QEMU. Meanwhile, if the host backend is not
> device-dax, QEMU cannot guarantee the persistence of guest writes.
> Before solving this flushing problem, QEMU should warn users if the
> host backend is not device-dax.
>
> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> Message-id: CAPcyv4hV2-ZW8SMCRtD0P_86KgR3DHOvNe+6T5SY2u7wXg3gEg@mail.gmail.com
> ---
>  hw/mem/nvdimm.c      |  6 ++++++
>  include/qemu/osdep.h |  9 ++++++++
>  util/osdep.c         | 61 ++++++++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 76 insertions(+)
>
> diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
> index a9b0863f20..b23542fbdf 100644
> --- a/hw/mem/nvdimm.c
> +++ b/hw/mem/nvdimm.c
> @@ -26,6 +26,7 @@
>  #include "qapi/error.h"
>  #include "qapi/visitor.h"
>  #include "hw/mem/nvdimm.h"
> +#include "qemu/error-report.h"
>
>  static void nvdimm_get_label_size(Object *obj, Visitor *v, const char *name,
>                                    void *opaque, Error **errp)
> @@ -84,6 +85,11 @@ static void nvdimm_realize(PCDIMMDevice *dimm, Error **errp)
>      NVDIMMDevice *nvdimm = NVDIMM(dimm);
>      uint64_t align, pmem_size, size = memory_region_size(mr);
>
> +    if (!qemu_fd_is_dev_dax(memory_region_get_fd(mr))) {
> +        error_report("warning: nvdimm backend does not look like a DAX device, "
> +                     "unable to guarantee persistence of guest writes");
> +    }
> +
>      align = memory_region_get_alignment(mr);
>
>      pmem_size = size - nvdimm->label_size;
> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> index 1c9f5e260c..7f26af371e 100644
> --- a/include/qemu/osdep.h
> +++ b/include/qemu/osdep.h
> @@ -470,4 +470,13 @@ char *qemu_get_pid_name(pid_t pid);
>   */
>  pid_t qemu_fork(Error **errp);
>
> +/**
> + * qemu_fd_is_dev_dax:
> + *
> + * Check whether @fd describes a DAX device.
> + *
> + * Returns true if it is; otherwise, return false.
> + */
> +bool qemu_fd_is_dev_dax(int fd);
> +
>  #endif
> diff --git a/util/osdep.c b/util/osdep.c
> index a2863c8e53..02881f96bc 100644
> --- a/util/osdep.c
> +++ b/util/osdep.c
> @@ -471,3 +471,64 @@ writev(int fd, const struct iovec *iov, int iov_cnt)
>      return readv_writev(fd, iov, iov_cnt, true);
>  }
>  #endif
> +
> +#ifdef __linux__
> +static ssize_t qemu_dev_dax_sysfs_read(int fd, const char *entry,
> +                                       char *buf, size_t len)
> +{
> +    ssize_t read_bytes;
> +    struct stat st;
> +    unsigned int major, minor;
> +    char *path, *pos;
> +    int sysfs_fd;
> +
> +    if (fstat(fd, &st)) {
> +        return 0;
> +    }
> +
> +    major = major(st.st_rdev);
> +    minor = minor(st.st_rdev);
> +    path = g_strdup_printf("/sys/dev/char/%u:%u/%s", major, minor, entry);
> +
> +    sysfs_fd = open(path, O_RDONLY);
> +    g_free(path);
> +    if (sysfs_fd == -1) {
> +        return 0;
> +    }
> +
> +    read_bytes = read(sysfs_fd, buf, len - 1);
> +    close(sysfs_fd);
> +    if (read_bytes > 0) {
> +        buf[read_bytes] = '\0';
> +        pos = g_strstr_len(buf, read_bytes, "\n");
> +        if (pos) {
> +            *pos = '\0';
> +        }
> +    }
> +
> +    return read_bytes;
> +}
> +#endif /* __linux__ */
> +
> +bool qemu_fd_is_dev_dax(int fd)
> +{
> +    bool is_dax = false;
> +
> +#ifdef __linux__
> +    char devtype[7];
> +    ssize_t len;
> +
> +    if (fd == -1) {
> +        return false;
> +    }
> +
> +    len = qemu_dev_dax_sysfs_read(fd, "device/devtype",
> +                                  devtype, sizeof(devtype));
> +    if (len <= 0) {
> +        return false;
> +    }
> +    is_dax = !strncmp(devtype, "nd_dax", len);

There's no guarantee that device-dax instances are always parented by
an "nd_dax" device-type. A more reliable check is to see if
"/sys/dev/char/%u:%u/subsystem" points to "/sys/class/dax".