Re: Can we force a KVM VCPU in Guest Mode to Exit to User Mode From User Mode ?

["Mian M. Hamayun" <mian-muhammad.hamayun@imag.fr>]

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On 07/26/2012 12:06 PM, Avi Kivity wrote:
> On 07/26/2012 12:38 PM, Mian M. Hamayun wrote:
>
> <snip>
>
>> This mechanism 'seems' to work fine when both vcpu threads are in User
>> Mode. But when booting an SMP Guest, the boot processor (BSP) initially
>> executes the bootstrap code while the non-boot processors (APs) are
>> waiting for initial INIT-SIPI-SIPI messages.
>>
>> What I fail to understand is if an AP is currently waiting for an INIT
>> signal, and we call the "run_on_cpu" function above for this cpu, it
>> blocks the whole system, as the AP is in Guest mode and cannot call the
>> "flush_queued_work" and the BSP is waiting for this to happen.
>>
>> How can we resolve this deadlock ? Is there a way to force the AP to
>> quit the Guest Mode, by using some specific mechanism from the User mode ?
> When a vcpu is waiting for an INIT, it still responds to signals and
> will return to userspace if a signal is received.  Did you observe
> something different?
>

Hi Avi,

So it means that when we execute the following:

err = pthread_kill(env->thread, SIG_IPI);

then this VCPU thread should wake-up and force the VCPU to quit the 
guest mode ?
But I am not getting this behavior as the VCPU thread remains blocked in 
Guest Mode.
What could be wrong here ?

Many Thanks,
Hamayun

P.S. Please see the following trace; As it might help understanding the 
problem. (thread 2 is the BSP and thread 3 is for AP)

run_on_cpu: Kicked CPU#1 ... Waiting on qemu_work_cond

Program received signal SIGUSR1, User defined signal 1.
^C
Program received signal SIGINT, Interrupt.
0xb7fdd424 in __kernel_vsyscall ()
(gdb) info threads
   Id   Target Id         Frame
   3    Thread 0xaea17b40 (LWP 2843) "arch.x" 0xb7fdd424 in 
__kernel_vsyscall ()
   2    Thread 0xaf218b40 (LWP 2842) "arch.x" 0xb7fdd424 in 
__kernel_vsyscall ()
* 1    Thread 0xb7359940 (LWP 2822) "arch.x" 0xb7fdd424 in 
__kernel_vsyscall ()
(gdb) thread 2
[Switching to thread 2 (Thread 0xaf218b40 (LWP 2842))]
#0  0xb7fdd424 in __kernel_vsyscall ()
(gdb) bt
#0  0xb7fdd424 in __kernel_vsyscall ()
#1  0xb7f7096b in pthread_cond_wait@@GLIBC_2.3.2 () from 
/lib/i386-linux-gnu/libpthread.so.0
#2  0xb7fc5715 in qemu_cond_wait (cond=0xb7fda640, lock=0xb7fda670) at 
gdb_srv_arch.c:578
#3  0xb7fc5981 in run_on_cpu (env=0x8465818, func=0xb7fc5417 
<kvm_invoke_set_guest_debug>, data=0xaf20fe50) at gdb_srv_arch.c:782
#4  0xb7fc5b90 in kvm_update_guest_debug (env=0x8465818, 
reinject_trap=0) at gdb_srv_arch.c:863
#5  0xb7fc5ef1 in kvm_remove_all_breakpoints (current_env=0x8465418) at 
gdb_srv_arch.c:983
#6  0xb7fc2e0a in gdb_breakpoint_remove_all (env=0x8465418) at gdb_srv.c:369
#7  0xb7fc3139 in gdb_handle_packet (s=0x80ac3e0, line_buf=0x80ac3fc 
"?") at gdb_srv.c:497
#8  0xb7fc439c in gdb_read_byte (s=0x80ac3e0, ch=102) at gdb_srv.c:830
#9  0xb7fc4522 in gdb_loop (env=0x8465418) at gdb_srv.c:878
#10 0xb7fc4621 in gdb_srv_handle_debug (env=0x8465418) at gdb_srv.c:924
#11 0xb7fc5265 in kvm_arch_handle_debug (env=0x8465418) at 
gdb_srv_arch.c:424
#12 0xb7fac3a8 in kvm_cpu__start (cpu=0x8465418) at kvm-cpu.c:588
#13 0xb7fc6521 in kvm_cpu_thread (arg=0x8465418) at libkvm-main.c:276
#14 0xb7f6cd4c in start_thread () from /lib/i386-linux-gnu/libpthread.so.0
#15 0xb7d7bace in clone () from /lib/i386-linux-gnu/libc.so.6
(gdb) thread 3
[Switching to thread 3 (Thread 0xaea17b40 (LWP 2843))]
#0  0xb7fdd424 in __kernel_vsyscall ()
(gdb) bt
#0  0xb7fdd424 in __kernel_vsyscall ()
#1  0xb7d73869 in ioctl () from /lib/i386-linux-gnu/libc.so.6
#2  0xb7fabe9c in kvm_cpu__run (vcpu=0x8465818) at kvm-cpu.c:429
#3  0xb7fac376 in kvm_cpu__start (cpu=0x8465818) at kvm-cpu.c:578
#4  0xb7fc6521 in kvm_cpu_thread (arg=0x8465818) at libkvm-main.c:276
#5  0xb7f6cd4c in start_thread () from /lib/i386-linux-gnu/libpthread.so.0
#6  0xb7d7bace in clone () from /lib/i386-linux-gnu/libc.so.6




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