Re: [Qemu-devel] [PATCH 1/2] Dump: introduce a Filesystem in Userspace

[Daniel P. Berrange]

On Mon, May 09, 2016 at 06:20:22PM +0200, Petr Tesarik wrote:
> On Mon, 9 May 2016 17:13:07 +0100
> "Daniel P. Berrange" <address@hidden> wrote:
> 
> > On Mon, May 09, 2016 at 09:52:28AM -0600, Eric Blake wrote:
> > > On 05/07/2016 05:32 PM, Nan Li wrote:
> > > > When running the command "dump-guest-memory", we usually need a large 
> > > > space
> > > > of storage to save the dumpfile into disk. It costs not only much time 
> > > > to
> > > > save a file in some of hard disks, but also costs limited storage in 
> > > > host.
> > > > In order to reduce the saving time and make it convenient for users to 
> > > > dump
> > > > the guest memory, we introduce a Filesystem in Userspace (FUSE) to save 
> > > > the
> > > > dump file in RAM. It is selectable in the configure file, adding a 
> > > > compiling
> > > > of package "fuse-devel". It doesn't change the way of dumping guest 
> > > > memory.
> > > 
> > > Why introduce FUSE? Can we reuse NBD instead?
> > 
> > The commit message talks of letting QEMU dump to RAM avoiding disk I/O.
> > IOW, it seems like it could just dump to any tmpfs directory.
> > 
> > I'm not really seeing a compelling reason why QEMU needs to mount a fuse
> > filesystem itself - whatever app is using QEMU could handle mounting of
> > fs without QEMU's involvement at all.
> 
> The ultimate goal is to export internal QEMU state (memory content,
> register values) as an ELF file, so you could simply reuse any existing
> tools that can work with ELF dump files (gdb, crash, makedumpfile,
> readelf, etc.) instead of re-inventing the wheel for each of those
> tools.
> 
> This cannot be really done from outside of QEMU without too much
> overhead (how would you access guest memory from outside QEMU?).

Maybe I'm missing something, but IIUC the 'dump-guest-memory' monitor
command in QEMU already dumps in ELF format which can be used by standard
ELF tools. If you don't want that dump to hit disk, then you could mount
a tmpfs and then tell QEMU to write to that.

Regards,
Daniel
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