Re: [Xen-devel] Problem with BIOS timer interrupts

[Gary Grebus <ggrebus@xxxxxxxxxxxxxxx>]

On Tue, 2008-11-18 at 20:02 +0000, Keir Fraser wrote:
> On 18/11/08 18:50, "Gary Grebus" <ggrebus@xxxxxxxxxxxxxxx> wrote:
> 
> > It uses this to timeout waiting for a key press.  The expected interrupt
> > is from the BIOS timer implemented in rombios.  But in fact, the loop
> > hangs.  However, if I insert a nop instruction between the sti and hlt,
> > then things work as expected.
> > 
> > Is there something wrong with this sequence?  This happens on AMD, so
> > it's not a quirk of the real mode emulations on Intel.

Interestingly, the same problem and "fix" apply on Intel under
vmxassist.  But likely nobody cares about that anymore (and gPXE has
other problems with vmxassist).

> > 
> > I notice that in the gPXE code currently in xen-unstable, the path that
> > uses this code is patched out.
> 
> As a data point, I commented it out because the delay's annoying rather than
> because it caused a boot hang for me. I was testing on Intel though.
> 
> Inserting the nop is obviously bogus (I expect you're aware of that :-),
> since it raises the opportunity of a wakeup-waiting race. That it fixes this
> issue is very weird. I expect we have some issue to do with leaving an
> interrupt shadow during HLT emulation -- why this would only trigger in real
> mode I cannot guess.
> 
> Wei's erratum is not applicable, for three reasons:
>  1. We disable C1 clock ramping
>  2. We always intercept HLT
>  3. STI; HLT is a standard x86 idiom used in all OSes, and this is the only
> place we're seeing a problem. Also the erratum would lead to rare
> non-deterministic hangs, not a hang every time (which is what you're
> seeing?).

OK.  It does appear to be family 0xf processor.  dom0 says:
processor       : 0
vendor_id       : AuthenticAMD
cpu family      : 15
model           : 65
model name      : Dual-Core AMD Opteron(tm) Processor 2216
stepping        : 2
cpu MHz         : 2394.000
cache size      : 1024 KB

> 
> I would say it's a good idea to see if you can repro this on xen-unstable.

OK... My usual xen-unstable setup is out of commission at the moment,
but I will try to reproduce it.

        /gary


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