Re: [Xenomai-core] Long execution time inside __ipipe_unstall_iret_root()

[Jan Kiszka <jan.kiszka@domain.hid>]

Jeroen Van den Keybus wrote:
>> Ouch, this shouldn't be allowed in user space! WBINVD is a privileged
>> instruction. Do we leak privileges to user land??? Please check if your
>> execution mode (privilege ring) is correct there.
> 
> 
> No, I rather meant a kernel-mode program that was controlled from the user
> space. Sorry for upsetting you.

Puh... fine, the world is round again. :)

> 
> But indeed, wbinvd is devastating if you can execute it, causing
>> typically around 300 us latencies, at worst even milliseconds
>> (cache-size and state dependent)!
> 
> 
> If I recall correctly, some of the Linux AGP GART drivers use(d?) it.

Yep, some only require this during setup (while MTRR reconfig e.g.),
others also use it during runtime of X - making your graphical system a
real-time killer.

> 
> This raises another interesting question: to what extent is the x86 actually
> a viable and dependable realtime platform, with its SMI's and highly
> uncontrollable caching architecture ? How would VT-based solutions compare ?

No surprise: x86 is a real-time minefield. Actually, I have been told
that PCI Express also requires cache flushes before starting DMA
transactions, but I haven't checked this yet.

Regarding fun with virtualisation over RT:

http://thread.gmane.org/gmane.comp.emulators.kvm.devel/8520/focus=8540
(There are even more threads about this, and some light at the end of
this tunnel, also for Intel. Check the archive.)

This reminds me that I still need to post my experimental kvm-over-ipipe
patch for VMX (yet Intel only, no AMD at hand :( ).

> (BTW Intel should really have implemented a feature to use parts of the
> cache as SRAM.)

Ack.

Jan

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