Re: [RFC][PATCH] new timeofday core subsystem (v. A3)

[Albert Cahalan <albert@users.sf.net>]

On Mon, 2005-03-14 at 12:27 -0800, Matt Mackall wrote:
> On Mon, Mar 14, 2005 at 12:04:07PM -0800, john stultz wrote:
> > > > > > > > +static inline cycle_t read_timesource(struct timesource_t* ts)
> > > > > > > > +{
> > > > > > > > +	switch (ts->type) {
> > > > > > > > +	case TIMESOURCE_MMIO_32:
> > > > > > > > +		return (cycle_t)readl(ts->mmio_ptr);
> > > > > > > > +	case TIMESOURCE_MMIO_64:
> > > > > > > > +		return (cycle_t)readq(ts->mmio_ptr);
> > > > > > > > +	case TIMESOURCE_CYCLES:
> > > > > > > > +		return (cycle_t)get_cycles();
> > > > > > > > +	default:/* case: TIMESOURCE_FUNCTION */
> > > > > > > > +		return ts->read_fnct();
> > > > > > > > +	}
> > > > > > > > +}
> > > Well where we'd read an MMIO address, we'd simply set read_fnct to
> > > generic_timesource_mmio32 or so. And that function just does the read.
> > > So both that function and read_timesource become one-liners and we
> > > drop the conditional branches in the switch.
> > 
> > However the vsyscall/fsyscall bits cannot call in-kernel functions (as
> > they execute in userspace or a sudo-userspace). As it stands now in my
> > design TIMESOURCE_FUNCTION timesources will not be usable for
> > vsyscall/fsyscall implementations, so I'm not sure if that's doable.
> > 
> > I'd be interested you've got a way around that.
> 
> We can either stick all the generic mmio timer functions in the
> vsyscall page (they're tiny) or leave the vsyscall using type/ptr but
> have the kernel internally use only the function pointer. Someone
> who's more familiar with the vsyscall timer code should chime in here.

When the vsyscall page is created, copy the one needed function
into it. The kernel is already self-modifying in many places; this
is nothing new.