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Re: [Qemu-devel] [PATCH 1/2] nvdimm: warn if the backend is not a DAX de
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH 1/2] nvdimm: warn if the backend is not a DAX device |
Date: |
Tue, 30 May 2017 10:15:11 +0100 |
User-agent: |
Mutt/1.8.0 (2017-02-23) |
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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