Re: [Qemu-devel] [PATCH v4 0/6] nvdimm: support MAP_SYNC for memory-backend-file

[Haozhong Zhang]

On 01/31/18 14:25 -0800, Dan Williams wrote:
> On Tue, Jan 30, 2018 at 10:02 PM, Haozhong Zhang
> <address@hidden> wrote:
> > Linux 4.15 introduces a new mmap flag MAP_SYNC, which can be used to
> > guarantee the write persistence to mmap'ed files supporting DAX (e.g.,
> > files on ext4/xfs file system mounted with '-o dax').
> 
> Wait, MAP_SYNC does not guarantee persistence. It makes sure that the
> metadata is in sync after a fault. However, that does not make
> filesystem-DAX safe for use with QEMU, because we still need to
> coordinate DMA with fileystem operations. There is no way to do that
> coordination from within a guest. QEMU needs to use device-dax if the
> guest might ever perform DMA to a virtual-pmem range. See this patch
> set for more details on the DAX vs DMA problem [1]. I think we need to
> enforce this in the host kernel. I.e. do not allow file backed DAX
> pages to be mapped in EPT entries unless / until we have a solution to
> the DMA synchronization problem. Apologies for not noticing this
> earlier.

QEMU does not truncate or punch holes of the file once it has been
mmap()'ed. Does the problem [1] still exist in such case?

Thanks,
Haozhong

> 
> [1]: https://lists.01.org/pipermail/linux-nvdimm/2017-December/013704.html