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Re: [Qemu-devel] [PATCH v2 2/4] nvdimm: warn if the backend is not a DAX
From: |
Dan Williams |
Subject: |
Re: [Qemu-devel] [PATCH v2 2/4] nvdimm: warn if the backend is not a DAX device |
Date: |
Tue, 6 Jun 2017 10:59:07 -0700 |
On Tue, Jun 6, 2017 at 12:22 AM, Haozhong Zhang
<address@hidden> 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 <address@hidden>
> Message-id: address@hidden
> ---
> 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".
Re: [Qemu-devel] [PATCH v2 3/4] nvdimm: add a boolean option "restrict", Michael S. Tsirkin, 2017/06/08
[Qemu-devel] [PATCH v2 1/4] nvdimm: add a macro for property "label-size", Haozhong Zhang, 2017/06/06
[Qemu-devel] [PATCH v2 2/4] nvdimm: warn if the backend is not a DAX device, Haozhong Zhang, 2017/06/06