[Qemu-devel] Re: [PATCH comment tweaked] msix: allow byte and word reading from mmio

[Michael S. Tsirkin]

On Tue, Nov 16, 2010 at 05:43:06PM +0100, Bernhard Kohl wrote:
> Am 16.11.2010 14:14, schrieb address@hidden:
> >Although explicitly disallowed by the PCI spec, some guests read a
> >single byte or word from mmio.  Likely a guest OS bug, but I have an OS
> >which reads single bytes and it works fine on real hardware.
> >
> >Signed-off-by: Bernhard Kohl<address@hidden>
> >Signed-off-by: Michael S. Tsirkin<address@hidden>
> >---
> >
> >OK so it could like something like the below.
> 
> Yes, this looks good for me.
> 
> >  However, my question is:
> >do we need to put this in or can the guest simply be fixed?
> 
> I tried to locate the code where the readw occurs,
> but not successful. It only occurs during booting our OS,
> and the virtio-net driver seems to be OK. With 4 virtio
> NICs we have the following readw accesses, thats all!
> 3 accesses per NIC and the first NIC appears twice.
> 
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000008
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000018
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000028
> MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000008
> MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000018
> MSI-X: msix_mmio_readw dev=0x9772c40 addr=0000000000000028
> MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000008
> MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000018
> MSI-X: msix_mmio_readw dev=0x977dc38 addr=0000000000000028
> MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000008
> MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000018
> MSI-X: msix_mmio_readw dev=0x9788d90 addr=0000000000000028
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000008
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000018
> MSI-X: msix_mmio_readw dev=0x9767c58 addr=0000000000000028
> 
> Is it possible to add a stack back tace printing to the
> readw function?


See some words of wisdom from Marcelo (this appliues to qemu-kvm.git):

<mst> marcelot, there is a guest that misbehaves
<mst> marcelot, so we get an fprintf in qemu
<mst> marcelot, but we don't know which place in guest caused the
problem, no backtrace, etc
<marcelot> mst, print guest RIP there? 
<mst> marcelot, 1. how to?
<marcelot> mst, cpu_synchronize_state(cpu_single_env) + print (env->eip) 
<marcelot> s/env/cpu_single_env/
<mst> marcelot, 2. can we also make it so that when io completes, guest
stops with backtrace in gdb?
<marcelot> mst, see kvm_show_code in qemu-kvm-x86.c
<marcelot> mst, either add a breakpoint there, or call abort() and gdb
core later
<mst> marcelot, abort will exit qemu though. gdb core will give me qemu
backtrace, not guest one, right?
<mst> marcelot, I would like kvm to pretend it hit a breakpoint
<mst> marcelot, or a segfault ... something
<marcelot> mst, well, vmstop() then. you can see guest RIP from there.
<marcelot> mst, yes gdb core will give qemu backtrace. inject a #GP to
see guest backtrace (see kvm_put_vcpu_events in qemu-kvm-x86.c
<marcelot> )

-- 
MST