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Re: [Qemu-devel] QEMU patch to allow VM introspection via libvmi

From: Valerio Aimale
Subject: Re: [Qemu-devel] QEMU patch to allow VM introspection via libvmi
Date: Thu, 22 Oct 2015 12:11:45 -0600
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 
On 10/22/15 5:50 AM, Markus Armbruster wrote:

Valerio Aimale <address@hidden> writes:


On 10/21/15 4:54 AM, Markus Armbruster wrote:

Valerio Aimale <address@hidden> writes:


On 10/19/15 1:52 AM, Markus Armbruster wrote:

Valerio Aimale <address@hidden> writes:


On 10/16/15 2:15 AM, Markus Armbruster wrote:

address@hidden writes:


All-

I've produced a patch for the current QEMU HEAD, for libvmi to
introspect QEMU/KVM VMs.

Libvmi has patches for the old qeum-kvm fork, inside its source tree:
https://github.com/libvmi/libvmi/tree/master/tools/qemu-kvm-patch

This patch adds a hmp and a qmp command, "pmemaccess". When the
commands is invoked with a string arguments (a filename), it will open
a UNIX socket and spawn a listening thread.

The client writes binary commands to the socket, in the form of a c
structure:

struct request {
         uint8_t type;   // 0 quit, 1 read, 2 write, ... rest reserved
         uint64_t address;   // address to read from OR write to
         uint64_t length;    // number of bytes to read OR write
};

The client receives as a response, either (length+1) bytes, if it is a
read operation, or 1 byte ifit is a write operation.

The last bytes of a read operation response indicates success (1
success, 0 failure). The single byte returned for a write operation
indicates same (1 success, 0 failure).

So, if you ask to read 1 MiB, and it fails, you get back 1 MiB of
garbage followed by the "it failed" byte?

Markus, that appear to be the case. However, I did not write the
communication protocol between libvmi and qemu. I'm assuming that the
person that wrote the protocol, did not want to bother with over
complicating things.

https://github.com/libvmi/libvmi/blob/master/libvmi/driver/kvm/kvm.c

I'm thinking he assumed reads would be small in size and the price of
reading garbage was less than the price of writing a more complicated
protocol. I can see his point, confronted with the same problem, I
might have done the same.

All right, the interface is designed for *small* memory blocks then.

Makes me wonder why he needs a separate binary protocol on a separate
socket.  Small blocks could be done just fine in QMP.

The problem is speed. if one's analyzing the memory space of a running
process (physical and paged), libvmi will make a large number of small
and mid-sized reads. If one uses xp, or pmemsave, the overhead is
quite significant. xp has overhead due to encoding, and pmemsave has
overhead due to file open/write (server), file open/read/close/unlink
(client).

Others have gone through the problem before me. It appears that
pmemsave and xp are significantly slower than reading memory using a
socket via pmemaccess.

That they're slower isn't surprising, but I'd expect the cost of
encoding a small block to be insiginificant compared to the cost of the
network roundtrips.

As block size increases, the space overhead of encoding will eventually
bite.  But for that usage, the binary protocol appears ill-suited,
unless the client can pretty reliably avoid read failure.  I haven't
examined its failure modes, yet.


The following data is not mine, but it shows the time, in
milliseconds, required to resolve the content of a paged memory
address via socket (pmemaccess) , pmemsave and xp

http://cl.ly/image/322a3s0h1V05

Again, I did not produce those data points, they come from an old
libvmi thread.

90ms is a very long time.  What exactly was measured?

That is a fair question to ask. Unfortunately, I extracted  that data
plot from an old thread in some libvmi mailing list. I do not have the
data and code that produced it. Sifting through the thread, I can see
the code
was never published. I will take it upon myself to produce code that
compares timing - in a fair fashion - of libvmi doing an atomic
operation and a larger-scale operation (like listing running
processes)  via gdb, pmemaccess/socket, pmemsave, xp, and hopefully, a
version of xp that returns byte streams of memory regions base64 or
base85 encoded in json strings. I'll publish results and code.

However, given workload and life happening, it will be some time
before I complete that task.

No problem.  I'd like to have your use case addressed, but there's no
need for haste.


Thanks, Markus. Appreciate your help.


[...]

Also, the pmemsave commands QAPI should be changed to be usable with
64bit VM's

in qapi-schema.json

from

---
{ 'command': 'pmemsave',
      'data': {'val': 'int', 'size': 'int', 'filename': 'str'} }
---

to

---
{ 'command': 'pmemsave',
      'data': {'val': 'int64', 'size': 'int64', 'filename': 'str'} }
---

In the QAPI schema, 'int' is actually an alias for 'int64'.  Yes, that's
confusing.

I think it's confusing for the HMP parser too. If you have a VM with
8Gb of RAM and want to snapshot the whole physical memory, via HMP
over telnet this is what happens:

$ telnet localhost 1234
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
QEMU 2.4.0.1 monitor - type 'help' for more information
(qemu) help pmemsave
pmemsave addr size file -- save to disk physical memory dump starting
at 'addr' of size 'size'
(qemu) pmemsave 0 8589934591 "/tmp/memorydump"
'pmemsave' has failed: integer is for 32-bit values
Try "help pmemsave" for more information
(qemu) quit

Your change to pmemsave's definition in qapi-schema.json is effectively a
no-op.

Your example shows *HMP* command pmemsave.  The definition of an HMP
command is *independent* of the QMP command.  The implementation *uses*
the QMP command.

QMP pmemsave is defined in qapi-schema.json as

       { 'command': 'pmemsave',
         'data': {'val': 'int', 'size': 'int', 'filename': 'str'} }

Its implementation is in cpus.c:

       void qmp_pmemsave(int64_t addr, int64_t size, const char *filename,
                         Error **errp)

Note the int64_t size.

HMP pmemsave is defined in hmp-commands.hx as

       {
           .name       = "pmemsave",
           .args_type  = "val:l,size:i,filename:s",
           .params     = "addr size file",
           .help       = "save to disk physical memory dump starting at 'addr' of 
size 'size'",
           .mhandler.cmd = hmp_pmemsave,
       },

Its implementation is in hmp.c:

       void hmp_pmemsave(Monitor *mon, const QDict *qdict)
       {
           uint32_t size = qdict_get_int(qdict, "size");
           const char *filename = qdict_get_str(qdict, "filename");
           uint64_t addr = qdict_get_int(qdict, "val");
           Error *err = NULL;

           qmp_pmemsave(addr, size, filename, &err);
           hmp_handle_error(mon, &err);
       }

Note uint32_t size.

Arguably, the QMP size argument should use 'size' (an alias for
'uint64'), and the HMP args_type should use 'size:o'.

Understand all that. Indeed, I've re-implemented 'pmemaccess' the same
way pmemsave is implemented. There is a single function, and two
points of entrance, one for HMP and one for QMP. I think pmemacess
mimics pmemsave closely.

However, if one wants to simply dump a memory region, via HMP for
human easy of use/debug/testing purposes, one cannot dump memory
regions that resides higher than 2^32-1

Can you give an example?

Yes. I was trying to dump the full extent of physical memory of a VM
that has 8GB memory space (ballooned). I simply did this:

$ telnet localhost 1234
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
QEMU 2.4.0.1 monitor - type 'help' for more information
(qemu) pmemsave 0 8589934591 "/tmp/memsaved"
'pmemsave' has failed: integer is for 32-bit values

Maybe I misunderstood how pmemsave works. Maybe I should have used
dump-guest-memory

This is am unnecessary limitation caused by 'size:i' instead of
'size:o'.  Fixable.
I think I tried changing size:i to size:l, but, I was still receiving the error.
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