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Re: [Qemu-devel] race between kvm-kmod-3.0 and kvm-kmod-3.3 // was: race

From: Peter Lieven
Subject: Re: [Qemu-devel] race between kvm-kmod-3.0 and kvm-kmod-3.3 // was: race condition in qemu-kvm-1.0.1
Date: Tue, 03 Jul 2012 15:01:30 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20110921 Thunderbird/3.1.15

Further output from my testing.

Linux 2.6.38 with included kvm module
Linux 3.0.0 with included kvm module

Linux 3.2.0 with included kvm module
Linux 2.6.28 with kvm-kmod 3.4
Linux 3.0.0 with kvm-kmod 3.4
Linux 3.2.0 with kvm-kmod 3.4

I can trigger the race with any of qemu-kvm 0.12.5, 1.0 or 1.0.1.
It might be that the code was introduced somewhere between 3.0.0
and 3.2.0 in the kvm kernel module and that the flaw is not
in qemu-kvm.

Any hints?


On 02.07.2012 17:05, Avi Kivity wrote:
On 06/28/2012 12:38 PM, Peter Lieven wrote:
does anyone know whats that here in handle_mmio?

     /* hack: Red Hat 7.1 generates these weird accesses. */
     if ((addr>  0xa0000-4&&  addr<= 0xa0000)&&  kvm_run->mmio.len == 3)
         return 0;

Just what it says.  There is a 4-byte access to address 0x9ffff.  The
first byte lies in RAM, the next three bytes are in mmio.  qemu is
geared to power-of-two accesses even though x86 can generate accesses to
any number of bytes between 1 and 8.

It appears that this has happened with your guest.  It's not impossible
that it's genuine.

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