Commit 04c44a08 authored Mar 17, 2008 by Jeremy Fitzhardinge Committed by Ingo Molnar Mar 27, 2008

xen: fix RMW when unmasking events



xen_irq_enable_direct and xen_sysexit were using "andw $0x00ff,
XEN_vcpu_info_pending(vcpu)" to unmask events and test for pending ones
in one instuction.

Unfortunately, the pending flag must be modified with a locked operation
since it can be set by another CPU, and the unlocked form of this
operation was causing the pending flag to get lost, allowing the processor
to return to usermode with pending events and ultimately deadlock.

The simple fix would be to make it a locked operation, but that's rather
costly and unnecessary.  The fix here is to split the mask-clearing and
pending-testing into two instructions; the interrupt window between
them is of no concern because either way pending or new events will
be processed.

This should fix lingering bugs in using direct vcpu structure access too.

[ Stable: needed in 2.6.24.x ]

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Cc: Stable <stable@kernel.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 5abbcf29

arch/x86/xen/enlighten.c

+1 −1

Original line number	Diff line number	Diff line
		@@ -95,7 +95,7 @@ struct shared_info HYPERVISOR_shared_info = (void )&dummy_shared_info;
		*
		* 0: not available, 1: available
		*/
		static int have_vcpu_info_placement = 0;
		static int have_vcpu_info_placement = 1;

		static void __init xen_vcpu_setup(int cpu)
		{

arch/x86/xen/xen-asm.S

+7 −2

Original line number	Diff line number	Diff line
		@@ -33,12 +33,17 @@
		events, then enter the hypervisor to get them handled.
		*/
		ENTRY(xen_irq_enable_direct)
		/* Clear mask and test pending */
		andw $0x00ff, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
		/* Unmask events */
		movb $0, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_mask

		/* Preempt here doesn't matter because that will deal with
		any pending interrupts. The pending check may end up being
		run on the wrong CPU, but that doesn't hurt. */

		/* Test for pending */
		testb $0xff, PER_CPU_VAR(xen_vcpu_info)+XEN_vcpu_info_pending
		jz 1f

		2: call check_events
		1:
		ENDPATCH(xen_irq_enable_direct)

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