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

[Arnabjyoti Kalita]

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







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
>
>