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

[Dan Williams]

On Thu, May 25, 2017 at 7:32 PM, 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.

I think this needs to be stronger than a "warn" it needs to be
explicitly forbidden when it is known to be unsafe.

>
> Signed-off-by: Haozhong Zhang <address@hidden>
> Message-id: address@hidden
> ---
> Cc: "Michael S. Tsirkin" <address@hidden>
> Cc: Igor Mammedov <address@hidden>
> Cc: Xiao Guangrong <address@hidden>
> Cc: Stefan Hajnoczi <address@hidden>
> Cc: Dan Williams <address@hidden>
> ---
> 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.