[Qemu-devel] Re: [PATCH 07/18] kvm: Add MCE signal support for !CONFIG_IOTHREAD

[Marcelo Tosatti]

On Wed, Jan 26, 2011 at 09:09:25AM +0100, Jan Kiszka wrote:
> On 2011-01-24 13:36, Jan Kiszka wrote:
> > On 2011-01-24 12:17, Marcelo Tosatti wrote:
> >> On Mon, Jan 10, 2011 at 09:32:00AM +0100, Jan Kiszka wrote:
> >>> From: Jan Kiszka <address@hidden>
> >>>
> >>> Currently, we only configure and process MCE-related SIGBUS events if
> >>> CONFIG_IOTHREAD is enabled. Fix this by factoring out the required
> >>> handler registration and system configuration. Make sure that events
> >>> happening over a VCPU context in non-threaded mode get dispatched as
> >>> VCPU MCEs.
> >>>
> >>> We also need to call qemu_kvm_eat_signals in non-threaded mode now, so
> >>> move it (unmodified) and add the required Windows stub.
> >>>
> >>> Signed-off-by: Jan Kiszka <address@hidden>
> >>> CC: Huang Ying <address@hidden>
> >>> ---
> >>>  cpus.c |  200 
> >>> +++++++++++++++++++++++++++++++++++++++------------------------
> >>>  1 files changed, 124 insertions(+), 76 deletions(-)
> >>>
> >>> diff --git a/cpus.c b/cpus.c
> >>> index 6da0f8f..b6f1cfb 100644
> >>> --- a/cpus.c
> >>> +++ b/cpus.c
> >>> @@ -34,9 +34,6 @@
> >>>  
> >>>  #include "cpus.h"
> >>>  #include "compatfd.h"
> >>> -#ifdef CONFIG_LINUX
> >>> -#include <sys/prctl.h>
> >>> -#endif
> >>>  
> >>>  #ifdef SIGRTMIN
> >>>  #define SIG_IPI (SIGRTMIN+4)
> >>> @@ -44,10 +41,24 @@
> >>>  #define SIG_IPI SIGUSR1
> >>>  #endif
> >>>  
> >>
> >>> @@ -912,6 +954,8 @@ static int qemu_cpu_exec(CPUState *env)
> >>>  
> >>>  bool cpu_exec_all(void)
> >>>  {
> >>> +    int r;
> >>> +
> >>>      if (next_cpu == NULL)
> >>>          next_cpu = first_cpu;
> >>>      for (; next_cpu != NULL && !exit_request; next_cpu = 
> >>> next_cpu->next_cpu) {
> >>> @@ -923,7 +967,11 @@ bool cpu_exec_all(void)
> >>>          if (qemu_alarm_pending())
> >>>              break;
> >>>          if (cpu_can_run(env)) {
> >>> -            if (qemu_cpu_exec(env) == EXCP_DEBUG) {
> >>> +            r = qemu_cpu_exec(env);
> >>> +            if (kvm_enabled()) {
> >>> +                qemu_kvm_eat_signals(env);
> >>> +            }
> >>> +            if (r == EXCP_DEBUG) {
> >>>                  break;
> >>>              }
> >>
> >> SIGBUS should be processed outside of vcpu execution context, think of a
> >> non MCE SIGBUS while vm is stopped. Could use signalfd for that.
> > 
> > signalfd - that's the missing bit. I was thinking of how to handle
> > SIGBUS events raised outside the vcpu context. We need to handle them
> > synchronously, and signalfd should allow this.
> 
> This was straightforward. But now I wonder what actually makes this
> pattern work. Doesn't the kernel force-inject SIGBUS, i.e. ignores any
> blocking? Or does this only apply to BUS_MCEERR_AR?

SIGBUS is only forced if BUS_MCEERR_AR and the poisoned memory was not accessed 
on behalf of the guest (say directly by qemu).

> >> But the SIGBUS handler for !IOTHREAD case should not ignore Action
> >> Required, since it might have been generated in vcpu context.
> >>
> > 
> > Yes, the sigbus handler will require some rework when we actually start
> > using it for !IOTHREAD.
> 
> And this no longer makes sense to me. The current version simply uses
> the same sigbus handler for both modes, an that forwards the error code
> properly. What did you mean?

There are two handlers, kvm_on_sigbus and kvm_on_sigbus_vcpu.
kvm_on_sigbus, the handler for iothread, dies on BUS_MCEERR_AR (which
will be generated if poisoned memory is accessed on behalf of vcpu). It
should be handled with !CONFIG_IOTHREAD.