Subject: Re: linux-next: add utrace tree

From: Ananth N Mavinakayanahalli <[email-masked]>
Date: Fri, 29 Jan 2010 10:25:46 +0530

On Thu, Jan 28, 2010 at 09:55:02AM +0100, Ingo Molnar wrote:

...

> Lets compare the two cases via a drawing. Your current uprobes submission 
> does:
> 
>  [kernel]      do probe thing     single-step trap
>                ^            |     ^              |
>                |            v     |              v
>  [user]     INT3            XOL-ins              next ins-stream
> 
>  ( add the need for serialization to make sure the whole single-step thing 
>    does not get out of sync with reality. )
> 
> And emulator approach would do:
> 
>  [kernel]      emul-demux-fastpath, do probe thing
>                ^                                 |
>                |                                 v
>  [user]     INT3                                 next ins-stream
> 
> far simpler conceptually, and faster as well, because it's one kernel entry.

Ingo,

Yes, conceptually, emulation is simpler. In fact, it may even be the
right thing to do from a housekeeping POV if gdb were enabled to use
breakpoint assistance in the kernel. However... emulation is not
easy. Just quoting Peter Anvin:

> On the more general rule of interpretation: I'm really concerned about
> having a bunch of partially-capable x86 interpreters all over the
> kernel.  x86 is *hard* to emulate, and it will only get harder as the
> architecture evolves.
>
>       -hpa

Yes, I know you suggested we start with a small subset.

We already have an implementation of instruction emulation in kernel for
x86 and powerpc, but its too KVM centric. If there is a generic
emulation layer, we would use it.

There are conflicting opinions for either case; complicated as it is,
the XOL scheme works and, to a large extent, it is easily extendable to
other architectures compared to the emulation approach. Uprobes can be
made to use emulation when possible/available, but I don't think this
should be gating decision for the initial implementation of the feature.

Ananth