[Crash-utility] PATCH 00/10] teach crash to work with "live" ramdump

[Dave Anderson]

----- Original Message -----
> On 04/26, Dave Anderson wrote:
> >
> > I didn't write this code, but here's how I understand it.
> 
> Thanks Dave, I'll read your explanation tomorrow, I need to run away again.
> 
> > > OK. So I am running the rhel7 guest on my Fedora machine and I pass the following
> > > (additional) options to qemu:
> > >
> > > 	-object memory-backend-file,id=MEM,size=128m,mem-path=/tmp/MEM,share=on \
> > > 	-numa node,memdev=MEM \
> > >
> > > so in this (trivial) case /tmp/MEM represents the physical memory as it seen by
> > > the guest.
> >
> > Exactly what is this /tmp/MEM that you speak of?
> 
> please note the "mem-path=/tmp/MEM" in the memory-backend-file arg above.
> 
> With this option qemu doesn't use the anonymous/private mapping for the guest's
> physical memory it creates a file (specified by mem-path=). The host can read
> (and write of course) to this file. This file _is_ the guest's physical memory.
> 
> Just in case, you pass multiple memory-backend-file/numa arguments, so you will
> have the "multi-file" ramdump.
> 
> > > Now suppose that this guest crashes and qemu exits. In this case the "live" mode
> > > makes no sense, if nothing else it is slower.
> >
> > I don't understand.  Does the /tmp/MEM file still exist somewhere after the guest
> > crashed?
> 
> Yes,
> 
> >
> > > "live" ramdump is a bit more interesting. I can do
> > >
> > > 	$ crash path-to-rhel7-vmlinux live:/tmp/MEM at 0
> >
> > Again, I am clueless as to what /tmp/MEM consists of on the guest.
> 
> See above,
> 
> > Is is a pseudo-file
> 
> No, just a regular file, qemu creates it and does mmap(MAP_SHARED) on it.
> 
> > that constantly contains the
> > current contents of the guest's physical memory?
> 
> Yes,
> 
> > Is it like /dev/mem?
> 
> yes, but more like /dev/crash.
> 
> Oleg.

Unfortunately I am completely unfamiliar with qemu option specifications.
So if I were to log into the guest machine, does a /tmp/MEM file exist?
Or does it exist on the host machine?

Dave