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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jun 12, 2017 at 11:18:21AM +0800, Haozhong Zhang wrote:
> On 06/08/17 15:51 +0300, Michael S. Tsirkin wrote:
> > On Tue, Jun 06, 2017 at 03:22:27PM +0800, Haozhong Zhang 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.
> > 
> > No one reads warnings unless things fail but they can be a debugging aid
> > if they do. But they have to be simple and robust to be helpful for
> > that. This one seems to me too complex and fragile.
> >
> > 
> > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > > Message-id: CAPcyv4hV2-ZW8SMCRtD0P_86KgR3DHOvNe+6T5SY2u7wXg3gEg@mail.gmail.com
> > 
> > Don't include this line in commit log please. Put it after --- if you
> > must.
> > 
> > > ---
> > >  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;
> > > +    }
> > 
> > This can easily trigger false positives e.g. if sysfs access
> > is blocked by selinux.
> >
> 
> A part of userland interface of nvdimm is exposed via sysfs, so QEMU
> has to access to certain sysfs entries in order to, e.g. perform the
> DAX check in this patch and get alignment requirement in patch 4.
> 
> I agree it's not robust if the permission is not properly granted. We
> may either
> 1) require users to grant the permissions to QEMU and document the
>    requirements
>  or
> 2) get the information from sysfs from the outside of QEMU
>    (e.g. libvirt) which has the permission and then pass to QEMU.
> 
> Which one do you think is better?

2) since that allows emulating things without hardware dax.

> > 
> > > +    is_dax = !strncmp(devtype, "nd_dax", len);
> > 
> > E.g. will return true on any string starting with nd_dax.
> > And it's not clear non-dax devices can't guarantee safety.
> 
> This check can be done by checking whether /sys/dev/char/MAJ:MIN/subsystem
> points to /sys/class/dax, as Dan suggested, if we decide to access
> sysfs in QEMU.
> 
> Thanks,
> Haozhong
> 
> > 
> > > +#endif /* __linux__ */
> > > +
> > > +    return is_dax;
> > > +}
> > > -- 
> > > 2.11.0