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

[Stefan Hajnoczi]

On Fri, May 26, 2017 at 10:24:37AM +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.
> 
> 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>
> ---
>  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")) {

Does not work with relative paths, symlinks, or device nodes in
non-standard locations.

A more accurate check is to fstat the file descriptor for the char
device major/minor number and verify that
/sys/dev/char/MAJ:MIN/device/devtype is "nd_dax".

> +        goto out;
> +    }
> +
> +    dev_name = mem_path + strlen("/dev/");
> +    sysfs_path = g_strdup_printf("/sys/class/dax/%s", dev_name);
> +    if (access(sysfs_path, F_OK)) {
> +        goto out;
> +    }
> +
> +    is_dax = true;

This check is Linux-specific, please use #ifdef __linux__.  It's okay to
always print an error on other operating systems.

> +
> + out:
> +    if (!is_dax) {
> +        error_report("warning: nvdimm backend %s is not DAX device, "

The check is not 100% accurate so:

s/is not/does not look like a/

> +                     "unable to guarantee persistence of guest writes",
> +                     mem_path ?: "RAM");
> +    }
> +
> +    g_free(sysfs_path);
> +    g_free(mem_path);
> +}
> +
>  static void nvdimm_realize(PCDIMMDevice *dimm, Error **errp)
>  {
>      MemoryRegion *mr = host_memory_backend_get_memory(dimm->hostmem, errp);
>      NVDIMMDevice *nvdimm = NVDIMM(dimm);
>      uint64_t align, pmem_size, size = memory_region_size(mr);
>  
> +    nvdimm_check_dax(dimm->hostmem);
> +
>      align = memory_region_get_alignment(mr);
>  
>      pmem_size = size - nvdimm->label_size;
> -- 
> 2.11.0
>

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