[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] do_iret bug in xen

On Tue, 2007-11-27 at 20:15 -0500, Ashish Bijlani wrote:
> current is extracted from per processor stack area. so are the
> registers. right? 


> do_iret gets current and regs from the processor's stack area. so does
> ret_from_intr. 

yes. but the latter from a different stack *frame* in the case you're
concerned about. same stack, though.

> so they both point to a fixed per processor stack area. there are not
> _different_ stack frames. 

i see what you mean. let's look at the stack when an IPI occurs to
trigger a context switch.


guest calls __HYPERVISOR_iret (saves guest cs)
        do_iret tinkers with guest_cpu_user_regs
        IPI caught (saves xen cs)

                ret_from_intr tests xen (!!) cs, not guest cs
        do_iret continues and finishes. same current.
        test_all_events calls do_softirq. schedules. new current.

return to new current.


UREGS_cs in entry.S refers to the interrupted context, not the saved
guest context. this may be the same: the IPI would have mattered if it
interrupted the guest. it did not. so the softirq will only be run
before returning to the guest, not in between.


Daniel Stodden
LRR     -      Lehrstuhl fÃr Rechnertechnik und Rechnerorganisation
Institut fÃr Informatik der TU MÃnchen             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
PGP Fingerprint: F5A4 1575 4C56 E26A 0B33  3D80 457E 82AE B0D8 735B

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.