[PATCH] arm/arm64: KVM: Complete WFI/WFE instructions

Christoffer Dall christoffer.dall at linaro.org
Tue Aug 26 05:49:47 PDT 2014

The architecture specifies that when the processor wakes up from a WFE
or WFI instruction, the instruction is considered complete, however we
currrently return to EL1 (or EL0) at the WFI/WFE instruction itself.

While most guests may not be affected by this because their local
exception handler performs an exception returning setting the event bit
or with an interrupt pending, some guests like UEFI will get wedged due
this little mishap.

Simply skip the instruction when we have completed the emulation.

Cc: Marc Zyngier <marc.zyngier at arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
Signed-off-by: Christoffer Dall <christoffer.dall at linaro.org>
Note that I haven't confirmed exactly how Linux guests survives WFI
without this fix.  My working hypothesis is that the Linux ISR doesn't
complete the interrupt, leaving an active interrupt on the LR, which may
cause the PE to consider it a wake-up event for the WFI despite the EL1
handler returning to the WFI instruction itself.  Tianocore on the other
hand may complete the interrupt entirely before returning to the WFI

Input on this is most welcome and likely to improve the quality of my

 arch/arm/kvm/handle_exit.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/arch/arm/kvm/handle_exit.c b/arch/arm/kvm/handle_exit.c
index 4c979d4..a96a804 100644
--- a/arch/arm/kvm/handle_exit.c
+++ b/arch/arm/kvm/handle_exit.c
@@ -93,6 +93,8 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
+	kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
 	return 1;

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