Re: 2.6.18-rc6-mm1: GPF loop on early boot

Ingo Molnar
2006-09-11T08:03:19Z

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> Ingo Molnar wrote:
> >btw., what's the connection of %gs based PDA to Xen and
> >paravirtualization in general - %esp based current is just as
> >Xen-friendly, or am i wrong? I guess there must be some connection,
> >given that you are working on this ;)
> >
>
> Yep. The goal is to put the Xen VCPU structure into the PDA, so that it
> can be easily accessed. At present, masking events (ie, cli), is
> something along the lines of
>
> xen_shared_info->vcpu[smp_processor_id()].mask = 1
>
> which comes out to something like 20 bytes of code, and is probably too
> awkward to inline. If the vcpu is in the PDA, it would come out to:
>
> movb $1, %gs:xen_vcpu_mask
>
> which has the added benefit of not needing a register.

take a look at lockdep: amongst other things it adds the TRACE_IRQFLAGS
/ irqflags.h infrastructure, which could easily be modified to allow the
easy replacement of cli/sti/pushf/popf. I wrote it with the side-goal of
paravirtualization. I.e. instead of the DISABLE_INTERRUPT /
ENABLE_INTERRUPT duplication you do in -mm currently please just enhance
irqflags.h to cover the needs of paravirtualization too. (most of which
should be the moving of cli/sti into the assembly callbacks)

into that the PDA would plug in a natural way: current->hardirqs_enabled
is basically just an alias for
!xen_shared_info->vcpu[smp_processor_id()].mask.

Ingo