Re: [Qemu-devel] [PATCH] SeaBios: Fix reset procedure reentrancy problem on qemu-kvm platform

[Kevin O'Connor <kevin@koconnor.net>]

On Sun, Dec 20, 2015 at 09:49:54AM +0000, Gonglei (Arei) wrote:
> > From: Kevin O'Connor [mailto:kevin@koconnor.net]
> > Sent: Saturday, December 19, 2015 11:12 PM
> > On Sat, Dec 19, 2015 at 12:03:15PM +0000, Gonglei (Arei) wrote:
> > > Maybe the root cause is not NMI but INTR, so yield() can open hardware
> > interrupt,
> > > And then execute interrupt handler, but the interrupt handler make the
> > SeaBIOS
> > > stack broken, so that the BSP can't execute the instruction and occur
> > exception,
> > > VM_EXIT to Kmod, which is an infinite loop. But I don't have any proofs except
> > > the surface phenomenon.
> > 
> > I can't see any reason why allowing interrupts at this location would
> > be a problem.
> > 
> Does it have any relationship with *extra stack* of SeaBIOS?

None that I can see.  Also, the kvm trace seems to show the code
trying to execute at rip=0x03 - that will crash long before the extra
stack is used.

> > > Kevin, can we drop yield() in smp_setup() ?
> > 
> > It's possible to eliminate this instance of yield, but I think it
> > would just push the crash to the next time interrupts are enabled.
> > 
> Perhaps. I'm not sure.
> 
> > > Is it really useful and allowable for SeaBIOS? Maybe for other components?
> > > I'm not sure. Because we found that when SeaBIOS is booting, if we inject a
> > > NMI by QMP, the guest will *stuck*. And the kvm tracing log is the same with
> > > the current problem.
> > 
> > If you apply the patches you had to prevent that NMI crash problem,
> > does it also prevent the above crash?
> > 
> Yes, but we cannot prevent the NMI injection (though I'll submit some patches to
> forbid users' NMI injection after NMI_EN disabled by RTC bit7 of port 0x70).
> 

-Kevin