Re: [Xen-devel] do_iret bug in xen

["Ashish Bijlani" <ashish.bijlani@xxxxxxxxx>]

yeah but the do_iret function is done on behalf of a guest, therefore do_iret function forces user cs and user ss

code excerpt 
"
    regs->rip    = iret_saved.rip;
    regs->cs     = iret_saved.cs | 3; /* force guest privilege */
    regs->rflags = (iret_saved.rflags & ~(EF_IOPL|EF_VM)) | EF_IE;
    regs->rsp    = iret_saved.rsp;
    regs->ss     = iret_saved.ss | 3; /* force guest privilege */
"
this can cause ret_from_intr go to test_all_events and finally go to __enter_scheduler

-a

On Nov 27, 2007 6:17 PM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:

On Tue, 2007-11-27 at 17:41 -0500, Ashish Bijlani wrote:
> "cpumask_raise_softirq(mask, SCHEDULE_SOFTIRQ)" sends an IPI to the
> target processor, this can cause current to change.

No it can't.

The IPI causes smp_event_check_interrupt() to be called which just ACKs
the IPI and returns via ret_from_intr (in entry.S). It is only if a
guest was interrupted that we go down the test_all_events path which
processes softirqs. If it was Xen which was interrupted then we go to
restore_all_xen which just returns to Xen. In this case Xen will
eventually return to the guest and take the test_all_events path and
process the softirq.

Ian.

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