Re: [Qemu-devel] Recording I/O activity after KVM does a VMEXIT

[Arnabjyoti Kalita]

Dear Pavel,

Thank you for providing me with all the details. Let us take an example of
a Network packet. In icount mode, when the network backend, receives a
network packet, you record the whole packet with the help of the
replay-filter. This packet will be written to the log file. Now when the
time comes for replay, you stop accepting any packets from the network
backend and directly inject all of the packets that you have already
recorded in the log file into the guest address space memory. Am I correct
in understanding this ?

Thanks and Regards,
Arnab

On Fri, Jun 1, 2018 at 1:31 AM, Pavel Dovgalyuk <address@hidden> wrote:

> Hi,
>
>
>
> I’m not familiar with KVM, but I know successful attempts of replaying the
> execution by logging IO and MMIO in TCG mode.
>
> The difference in CPU I/O and VM I/O is the following. In icount we record
> anything coming into the VM, but not into the CPU.
>
> It means that the whole packet is recorded. Virtual hardware behaves
> deterministically and therefore CPU will get identical
>
> input in case of replay, because the whole recorded packet is injected
> again by the filter.
>
>
>
> Pavel Dovgalyuk
>
>
>
> *From:* Arnabjyoti Kalita [mailto:address@hidden
> *Sent:* Thursday, May 31, 2018 11:14 PM
> *To:* Pavel Dovgalyuk
> *Cc:* Stefan Hajnoczi; address@hidden; Pavel Dovgalyuk
> *Subject:* Re: [Qemu-devel] Recording I/O activity after KVM does a VMEXIT
>
>
>
> Dear Pavel,
>
>
>
> Thank you for your answer. I am not being able to understand the
> difference between CPU I/Os and VM I/Os. Would any network packet that
> comes into the Guest OS from the outside be a part of VM I/O or CPU I/O ? I
> am only interested in "recording" and "replaying" those network packets
> that come from the outside into the networking backend and not the other
> way around. Say for example when I get a VMExit because of the arrival of a
> network packet, I will use the VMExit reason : "KVM_EXIT_MMIO"  to trace
> back to "e1000_mmio_write()" which I expect should be enough to record
> network packets that come from the outside and write to the guest address
> space for "e1000" devices. In such a case, I think I will not have to use
> the "network-filter" backend that you use to record VM I/O only. Let me
> know if you find errors in my approach.
>
>
>
> I will try to see how I can record disk packets. If disk packets use other
> ways of writing to the guest memory apart from a normal VMExit, I will try
> to find it out. Eventually I hope that it will use one of the available
> disk front-end functions to write to the guest memory from the disk, just
> like e1000 does with an "e1000_mmio_write()" call.
>
>
>
> Thanks and best regards,
>
> Arnab
>
>
>
>
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>
> On Thu, May 31, 2018 at 8:44 AM, Pavel Dovgalyuk <address@hidden>
> wrote:
>
> > From: Stefan Hajnoczi [mailto:address@hidden
> > On Wed, May 30, 2018 at 11:19:13PM -0400, Arnabjyoti Kalita wrote:
> > > I am trying to implement a 'minimal' record-replay mechanism for KVM,
> which
> > > is similar to the one existing for TCG via -icount. I am trying to
> record
> > > I/O events only (specifically disk and network events) when KVM does a
> > > VMEXIT. This has led me to the function kvm_cpu_exec where I can
> clearly
> > > see the different ways of handling all of the possible VMExit cases
> (like
> > > PIO, MMIO etc.). To record network packets, I am working with the e1000
> > > hardware device.
> > >
> > > Can I make sure that all of the network I/O, atleast for the e1000
> device
> > > happens through the KVM_EXIT_MMIO case and subsequent use of the
> > > address_space_rw() function ? Do I also need to look at other
> functions as
> > > well ? Also for recording disk activity, can I make sure that looking
> out
> > > for the KVM_EXIT_MMIO and/or KVM_EXIT_PIO cases in the vmexit
> mechanism,
> > > will be enough ?
> > >
> > > Let me know if there are other details that I need to take care of. I
> am
> > > using QEMU 2.11 on a x86-64 CPU and the guest runs a Linux Kernel 4.4
> with
> > > Ubuntu 16.04.
>
> The main icount-based record/replay advantage is that we don't record
> any CPU IO. We record only VM IO (e.g., by using the network filter).
>
> Disk devices may transfer data to CPU using DMA, therefore intercepting
> only VMExit cases will not be enough.
>
> Pavel Dovgalyuk
>
>
>