[Qemu-devel] Re: [PATCH v3 14/22] kvm: Fix race between timer signals and vcpu entry under !IOTHREAD

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-01-31 14:22, Avi Kivity wrote:
> On 01/31/2011 01:27 PM, Jan Kiszka wrote:
>> On 2011-01-31 11:03, Avi Kivity wrote:
>>>  On 01/27/2011 04:33 PM, Jan Kiszka wrote:
>>>>  Found by Stefan Hajnoczi: There is a race in kvm_cpu_exec between
>>>>  checking for exit_request on vcpu entry and timer signals arriving
>>>>  before KVM starts to catch them. Plug it by blocking both timer related
>>>>  signals also on !CONFIG_IOTHREAD and process those via signalfd.
>>>>
>>>>  As this fix depends on real signalfd support (otherwise the timer
>>>>  signals only kick the compat helper thread, and the main thread hangs),
>>>>  we need to detect the invalid constellation and abort configure.
>>>>
>>>>  Signed-off-by: Jan Kiszka<jan.kiszka@siemens.com>
>>>>  CC: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>>>>  ---
>>>>
>>>>  I don't want to invest that much into !IOTHREAD anymore, so let's see if
>>>>  the proposed catch&abort is acceptable.
>>>>
>>>
>>>  I don't understand the dependency on signalfd.  The normal way of doing
>>>  things, either waiting for the signal in sigtimedwait() or in
>>>  ioctl(KVM_RUN), works with SIGALRM just fine.
>>
>> And how would you be kicked out of the select() call if it is waiting
>> with a timeout? We only have a single thread here.
> 
> If we use signalfd() (either kernel provided or thread+pipe), we kick 
> out of select by select()ing it (though I don't see how it works without 
> an iothread, since an fd can't stop a vcpu unless you enable SIGIO on 
> it, which is silly for signalfd)
> 
> If you leave it as a naked signal, then it can break out of either 
> pselect() or vcpu.
> 
> Since the goal is to drop !CONFIG_IOTHREAD, the first path seems better, 
> I just don't understand the problem with emulated signalfd().
> 

With the emulated signalfd, there won't be any signal for the VCPU while
in KVM_RUN.

Jan

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