path: root/arch/i386/xen/enlighten.c
diff options
authorJeremy Fitzhardinge <jeremy@xensource.com>2007-07-17 18:37:07 -0700
committerJeremy Fitzhardinge <jeremy@goop.org>2007-07-18 08:47:46 -0700
commit9ec2b804e099e8a326369e6cccab10dee1d172ee (patch)
tree2626129bc8c8ce0be42290bb216517939be98f96 /arch/i386/xen/enlighten.c
parent600b2fc242992e552e0b4e24c8c1f084b341f39b (diff)
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 <jeremy@xensource.com>
Diffstat (limited to 'arch/i386/xen/enlighten.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/arch/i386/xen/enlighten.c b/arch/i386/xen/enlighten.c
index 4fa62a4cb7cc..9a8c1181c001 100644
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -838,6 +838,7 @@ void __init xen_setup_vcpu_info_placement(void)
paravirt_ops.irq_disable = xen_irq_disable_direct;
paravirt_ops.irq_enable = xen_irq_enable_direct;
paravirt_ops.read_cr2 = xen_read_cr2_direct;
+ paravirt_ops.iret = xen_iret_direct;

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