Re: [Qemu-devel] [RESEND PATCH 1/2] nvdimm: warn if the backend is not a DAX device

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 05/25/17 20:34 -0700, Dan Williams wrote:
> On Thu, May 25, 2017 at 7:32 PM, 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.
> 
> I think this needs to be stronger than a "warn" it needs to be
> explicitly forbidden when it is known to be unsafe.
>

I understand your worry and am not object to your suggestion, but
forbidden will change the existing behavior that allows such unsafe
usage. Let's wait for other maintainers' comments.

> >
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > Message-id: CAPcyv4hV2-ZW8SMCRtD0P_86KgR3DHOvNe+6T5SY2u7wXg3gEg@mail.gmail.com
> > ---
> > Cc: "Michael S. Tsirkin" <mst@redhat.com>
> > Cc: Igor Mammedov <imammedo@redhat.com>
> > Cc: Xiao Guangrong <guangrong.xiao@gmail.com>
> > Cc: Stefan Hajnoczi <stefanha@gmail.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > ---
> > Resend because the wrong maintainer email address was used.
> > ---
> >  hw/mem/nvdimm.c | 37 +++++++++++++++++++++++++++++++++++++
> >  1 file changed, 37 insertions(+)
> >
> > diff --git a/hw/mem/nvdimm.c b/hw/mem/nvdimm.c
> > index db896b0bb6..c7bb407f33 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)
> > @@ -78,12 +79,48 @@ static MemoryRegion *nvdimm_get_memory_region(PCDIMMDevice *dimm)
> >      return &nvdimm->nvdimm_mr;
> >  }
> >
> > +static void nvdimm_check_dax(HostMemoryBackend *hostmem)
> > +{
> > +    char *mem_path =
> > +        object_property_get_str(OBJECT(hostmem), "mem-path", NULL);
> > +    char *dev_name = NULL, *sysfs_path = NULL;
> > +    bool is_dax = false;
> > +
> > +    if (!mem_path) {
> > +        goto out;
> > +    }
> > +
> > +    if (!g_str_has_prefix(mem_path, "/dev/dax")) {
> > +        goto out;
> > +    }
> 
> What if we're using a symlink to a device-dax instance? The should
> explicitly be looking up the major / minor number of the device (after
> following any symlinks) and verifying that it is device-dax by
> checking /sys/dev/char/$major:$minor refers to a device-dax instance.
> 

I'll follow to your suggestion in the next version.

Thanks,
Haozhong