Re: 7.0-CURRENT Hang

In message <20060207182755.GB32998_at_comp.chem.msu.su>, Yar Tikhiy writes:
> On Tue, Feb 07, 2006 at 10:06:32AM -0800, Cy Schubert wrote:
> > In message <20060207173154.GE19674_at_comp.chem.msu.su>, Yar Tikhiy writes:
> > > On Mon, Feb 06, 2006 at 08:29:35PM -0800, Cy Schubert wrote:
> > > > 
> > > > On the Pentium P54C model (that's an old 120 MHz Pentium I use as a 4.x
> , 
> > > > 5.x, and 7.x ports build testbed) the CPUID instruction when called wit
> h AL
> > >  
> > > > = 0x02, CPUID returns EAX = EBX = ECX = EDX = 0. The code fragment in 
> > > > identcpu.c below results in "rounds" becoming 0xffffffff.
> > > > 
> > > > 	do_cpuid(0x2, regs);
> > > > 	rounds = (regs[0] & 0xff) - 1;
> > > > 
> > > > The subsequent loop of the following will loop virtually for ever (it t
> akes
> > >  
> > > > forever tor this machine to count down from 0xffffffff performing a ver
> y 
> > > > great many calls to get_INTEL_TLB in the process, virtually hanging the
>  
> > > > machine in the process.
> > > > 
> > > > 	while (rounds > 0) {
> > > > 		[... code ...]
> > > > 		rounds--;
> > > > 	}
> > > 
> > > FWIW, my presumably P54C machine (Family 5 Model 2 Stepping 6)
> > > doesn't indicate it has the CPUID 0x02 function.  That is, CPUID
> > > 0x00 returns EAX = 0x01, which is the highest function supported.
> > > Could you try to run the misc/cpuid port on your Pentium and show
> > > its output?  It might appear that the code around CPUID 0x02 shouldn't
> > > be reached at all in your case.  Zero values from CPUID 0x02 are
> > > pretty indicative of that.
> > 
> > Mine is Family 5 Model 2 Stepping 12. All of my doc is for Pentium-Pro and 
> > newer so you are probably correct.
> 
> Do you know what CPUID function 0x00 returns in EAX for your CPU?
> Hint: just run misc/cpuid once and show its output here.  I've just
> fixed the port so that it has no bogus dependencies and is very
> light-weight.

cwtest# /usr/local/bin/cpuid
 eax in    eax      ebx      ecx      edx
00000000 00000001 756e6547 6c65746e 49656e69
00000001 0000052c 00000000 00000000 000001bf

Vendor ID: "GenuineIntel"; CPUID level 1

Intel-specific functions:
Version 0000052c:
Type 0 - Original OEM
Family 5 - Pentium
Model 2 - 75-200
Stepping 12
Reserved 0


Feature flags: 000001bf:
FPU    Floating Point Unit
VME    Virtual 8086 Mode Enhancements
DE     Debugging Extensions
PSE    Page Size Extensions
TSC    Time Stamp Counter
MSR    Model Specific Registers
MCE    Machine Check Exception
CX8    COMPXCHG8B Instruction

cwtest#

> 
> > > Dealing with "rounds" equal to -1 can be a good idea anyway to catch
> > > braid dead CPUs instead of hanging the system on them.
> > 
> > Well, with rounds = -1 [actually (unsigned int)0xffffffff], the CPU will 
> > "appear" to hang as it "rounds" or loops virtually forever -- counting back
>  
> > from 0xffffffff on a 120 MHz machine and performing get TLB info a number 
> > of times each iteration takes hours to do just a few iterations. I've seen 
> > mine go through "rounds", decrementing rounds-- each time, for hours at a 
> > time, though initially before digging into it using DDB it did appear that 
> > the CPU was hung, it was just starting to loop for 4,294,967,295 times. On 
> > older and slower machines, if it took hours to iterate through a few 
> > iterations, my guess is that it would take days to loop through this code. 
> > My patch allows it to take the defaults and finally boot. If the CPU 
> > doesn't support AL = 0x02, what's the point of looping? It appears to run 
> > nicely with the patch.
> 
> I do see that rounds = -1 is causing trouble.
> I just meant that we should not call do_cpuid(0x02) at all if
> (cpu_high < 2) because it can result in undefined behavior.
> Your patch still makes sense because it deals with possible
> brain-dead CPUs.  I'd implement it in a slightly different
> way though -- stay tuned! :-)

Sounds OK by me.

Cheers,
Cy Schubert <Cy.Schubert_at_komquats.com>
Web:  http://www.komquats.com and http://www.bcbodybuilder.com
FreeBSD UNIX:  <cy_at_FreeBSD.org>   Web:  http://www.FreeBSD.org
BC Government:  <Cy.Schubert_at_gov.bc.ca>

    "Lift long enough and I believe arrogance is replaced by
    humility and fear by courage and selfishness by generosity
    and rudeness by compassion and caring."
        -- Dave Draper