From 9ec2b804e099e8a326369e6cccab10dee1d172ee Mon Sep 17 00:00:00 2001 From: Jeremy Fitzhardinge Date: Tue, 17 Jul 2007 18:37:07 -0700 Subject: xen: use iret directly when possible Most of the time we can simply use the iret instruction to exit the kernel, rather than having to use the iret hypercall - the only exception is if we're returning into vm86 mode, or from delivering an NMI (which we don't support yet). When running native, iret has the behaviour of testing for a pending interrupt atomically with re-enabling interrupts. Unfortunately there's no way to do this with Xen, so there's a window in which we could get a recursive exception after enabling events but before actually returning to userspace. This causes a problem: if the nested interrupt causes one of the task's TIF_WORK_MASK flags to be set, they will not be checked again before returning to userspace. This means that pending work may be left pending indefinitely, until the process enters and leaves the kernel again. The net effect is that a pending signal or reschedule event could be delayed for an unbounded amount of time. To deal with this, the xen event upcall handler checks to see if the EIP is within the critical section of the iret code, after events are (potentially) enabled up to the iret itself. If its within this range, it calls the iret critical section fixup, which adjusts the stack to deal with any unrestored registers, and then shifts the stack frame up to replace the previous invocation. Signed-off-by: Jeremy Fitzhardinge --- arch/i386/kernel/entry.S | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) (limited to 'arch/i386/kernel/entry.S') diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S index ffb236544270..32980b834935 100644 --- a/arch/i386/kernel/entry.S +++ b/arch/i386/kernel/entry.S @@ -1030,7 +1030,21 @@ ENTRY(xen_hypervisor_callback) CFI_ADJUST_CFA_OFFSET 4 SAVE_ALL TRACE_IRQS_OFF - mov %esp, %eax + + /* Check to see if we got the event in the critical + region in xen_iret_direct, after we've reenabled + events and checked for pending events. This simulates + iret instruction's behaviour where it delivers a + pending interrupt when enabling interrupts. */ + movl PT_EIP(%esp),%eax + cmpl $xen_iret_start_crit,%eax + jb 1f + cmpl $xen_iret_end_crit,%eax + jae 1f + + call xen_iret_crit_fixup + +1: mov %esp, %eax call xen_evtchn_do_upcall jmp ret_from_intr CFI_ENDPROC -- cgit v1.2.3