Re: [Qemu-devel] Are there plans to achieve ram live Snapshot feature?

[Wenchao Xia <xiawenc@linux.vnet.ibm.com>]

于 2013-8-12 19:33, Stefan Hajnoczi 写道:
> On Mon, Aug 12, 2013 at 12:26 PM, Alex Bligh <alex@alex.org.uk> wrote:
>> --On 12 August 2013 11:59:03 +0200 Stefan Hajnoczi <stefanha@gmail.com>
>> wrote:
>>
>>> The idea that was discussed on qemu-devel@nongnu.org uses fork(2) to
>>> capture the state of guest RAM and then send it back to the parent
>>> process.  The guest is only paused for a brief instant during fork(2)
>>> and can continue to run afterwards.
>>
>>
>> How would you capture the state of emulated hardware which might not
>> be in the guest RAM?
>
> Exactly the same way vmsave works today.  It calls the device's save
> functions which serialize state to file.
>
> The difference between today's vmsave and the fork(2) approach is that
> QEMU does not need to wait for guest RAM to be written to file before
> resuming the guest.
>
> Stefan
>
   I have a worry about what glib says:

"On Unix, the GLib mainloop is incompatible with fork(). Any program
using the mainloop must either exec() or exit() from the child without
returning to the mainloop. "

   There is another way to do it: intercept the write in kvm.ko(or other
kernel code). Since the key is intercept the memory change, we can do
it in userspace in TCG mode, thus we can add the missing part in KVM
mode. Another benefit of this way is: the used memory can be
controlled. For example, with ioctl(), set a buffer of a fixed size
which keeps the intercepted write data by kernel code, which can avoid
frequently switch back to user space qemu code. when it is full always
return back to userspace's qemu code, let qemu code save the data into
disk. I haven't check the exactly behavior of Intel guest mode about
how to handle page fault, so can't estimate the performance caused by
switching of guest mode and root mode, but it should not be worse than
fork().


-- 
Best Regards

Wenchao Xia