Re: [Xen-devel] Xen domU Timekeeping (a.k.a TSC/HPET issues)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

I'm afraid I don't know the answer to most of your questions (hence I'm
afraid I've trimmed the quotes rather aggressively) but here's some of
what I do know.

> But, practically, is there a safe CPU configuration? 

I think that part of the problem here is that it is very hard to
determine this at the hardware level. There are at least 3 (if not more)
CPUID feature bits which say "no really, the TSC is good and safe to use
this time, you can rely on that" because they keep inventing new ways to
get it wrong.

[...]
> 
> Since September, I can't find any further information about this
> issue. What is the state of this issue?  The inconsistency I see right
> now is this: in the July 2010 TSC discussion, a "Stefano Stabellini"
> posted this:
> 
> ====
> > /me wonders if timer_mode=1 is the default for xl?
> > Or only for xm?
> 
> no, it is not.
> Xl defaults to 0 [zero], I am going to change it right now.
> ====
> 
> So, it seems like (at least as of July 2010), xl is defaulting to
> "timer_mode=1".  That is, assuming that the then-current timer_mode is
> the same as present-day tsc_mode.

No, I believe they are different things.

tsc_mode is to do with the TSC, emulation vs direct exposure etc. Per
xen/include/asm-x86/time.h and (in recent xen-unstable) xl.cfg(5)

timer_mode is to do with the the way that timer interrupts are injected
into the guest. This is described in xen/include/public/hvm/params.h.
This isn't documented in xl.cfg(5) because I couldn't make head nor tail
of the meaning of that header :-(

>   In addition, I'm assuming he was changing it from 0 (zero) to 1
> (one)--and not some other mode.  But,
> 
>         xen-4.1.2/docs/misc/tscmode.txt

Remember that he was referring to timer_mode not tsc_mode...

> says:
> 
>         "The default mode (tsc_mode==0) checks TSC-safeness of the underlying
>         hardware on which the virtual machine is launched.  If it is
>         TSC-safe, rdtsc will execute at hardware speed; if it is not, rdtsc
>         will be emulated."
> 
> Which implies the default is always 0 (zero).  Which is it?

It seems that xl, in xen-unstable, defaults to:
        timer_mode = 1
        tsc_mode = 0
as does 4.1 as far as I can tell via code inspection.

> More importantly, is the solution to force tsc_mode=2?

IMHO this is safe in most situations unless you are running some sort of
workload (e.g. a well known database) which has stringent requirements
regarding the TSC for transactional consistency (hence the conservative
default).

>   If so, under what BIOS/xen-boot-params/dom0-boot-params conditions?
> And--please excuse my exasperation--but WTH does this have to do with
> ext3 versus ext4?  Is ext4 exquisitely sensitive to TSC/HPET
> "jumpiness" (if that's even what's happening)?

Sorry, I have no idea how/why the filesystem would be related to the
TSC.

It is possible you are actually seeing two bugs I suppose -- there have
been issues relating to ext4 and barriers in some kernel versions (I'm
afraid I don't recall the details, the list archives ought to contain
something).

Ian.




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