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Re: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH V13 5/5] docs: Added MAP_SYNC documentation
Date: Thu, 7 Feb 2019 22:00:46 -0500

On Fri, Feb 08, 2019 at 06:11:23PM +0800, Zhang, Yi wrote:
> From: Zhang Yi <address@hidden>
> Signed-off-by: Zhang Yi <address@hidden>
> ---
>  docs/nvdimm.txt | 25 ++++++++++++++++++++++---
>  qemu-options.hx |  5 +++++
>  2 files changed, 27 insertions(+), 3 deletions(-)
> diff --git a/docs/nvdimm.txt b/docs/nvdimm.txt
> index 5f158a6..a168429 100644
> --- a/docs/nvdimm.txt
> +++ b/docs/nvdimm.txt
> @@ -143,9 +143,28 @@ Guest Data Persistence
>  ----------------------
>  Though QEMU supports multiple types of vNVDIMM backends on Linux,
> -currently the only one that can guarantee the guest write persistence
> -is the device DAX on the real NVDIMM device (e.g., /dev/dax0.0), to
> -which all guest access do not involve any host-side kernel cache.
> +the only backend that can guarantee the guest write persistence is:
> +
> +A. DAX device (e.g., /dev/dax0.0, ) or
> +B. DAX file(mounted with dax option)
> +
> +both are use real NVDIMM device as backend, which supporting direct
> +access for files(no page cache).

So I would just drop above two lines.

How does one know that there's support for direct access?
it's just a question of kernel right?

And I don't think it matter wrt "real NVDIMM" - e.g. it
could be vNVDIMM in a nested virt scenario.
It will still survive host (i.e. L1 guest) crash.

Besides, the above two lines aren't grammatical.

> +
> +When using B (A file supporting direct mapping of persistent memory)
> +as a backend, write persistence is guaranteed if the host kernel has
> +support for the MAP_SYNC flag in the mmap system call (available
> +since Linux 4.15 and on certain distro kernels) and additionally
> +both 'pmem' and 'share' flags are set to 'on' on the backend.
> +
> +If these conditions are not satisfied i.e. if either 'pmem' or 'share'
> +are not set, if the backend file does not support DAX or if MAP_SYNC
> +is not supported by the host kernel, write persistence is not
> +guaranteed after a system crash. For compatibility reasons, these
> +conditions are silently ignored if not satisfied. Currently, no way
> +is provided to test for them.
> +For more details, please reference mmap(2) man page:
> +http://man7.org/linux/man-pages/man2/mmap.2.html.
>  When using other types of backends, it's suggested to set 'unarmed'
>  option of '-device nvdimm' to 'on', which sets the unarmed flag of the
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 08f8516..ef1da8f 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4002,6 +4002,11 @@ using the SNIA NVM programming model (e.g. Intel 
>  If @option{pmem} is set to 'on', QEMU will take necessary operations to
>  guarantee the persistence of its own writes to @option{mem-path}
>  (e.g. in vNVDIMM label emulation and live migration).
> +Also, we will map the backend-file with MAP_SYNC flag, which ensures the
> +file metadata is in sync for @option{mem-path} in case of host crash
> +or a power failure. MAP_SYNC requires support from both the host kernel
> +(since Linux kernel 4.15) and the filesystem of @option{mem-path} mounted
> +with DAX option.
>  @item -object 
> memory-backend-ram,address@hidden,address@hidden|off},address@hidden|off},address@hidden|off},address@hidden|off},address@hidden,address@hidden,address@hidden|preferred|bind|interleave}
> -- 
> 2.7.4

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