[PATCH 1/7] KVM: arm64: Update comment when skipping guest MMIO access instruction

[Will Deacon]

If a 32-bit guest accesses MMIO using a 16-bit Thumb-2 instruction that
is reported to the hypervisor without a valid syndrom (for example,
because of the addressing mode), then we may hand off the fault to
userspace. When resuming the guest, we unconditionally advance the PC
by 4 bytes, since ESR_EL2.IL is always 1 for data aborts generated without
a valid syndrome. This is a bit rubbish, but it's also difficult to see
how we can fix it without potentially introducing regressions in userspace
MMIO fault handling.

Update the comment when skipping a guest MMIO access instruction so that
this corner case is at least written down.

Cc: Marc Zyngier <maz at kernel.org>
Cc: Quentin Perret <qperret at google.com>
Signed-off-by: Will Deacon <will at kernel.org>
---
 arch/arm64/kvm/mmio.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
index 4e0366759726..b54ea5aa6c06 100644
--- a/arch/arm64/kvm/mmio.c
+++ b/arch/arm64/kvm/mmio.c
@@ -113,6 +113,13 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
 	/*
 	 * The MMIO instruction is emulated and should not be re-executed
 	 * in the guest.
+	 *
+	 * Note: If user space handled the emulation because the abort
+	 * symdrome information was not valid (ISV set in the ESR), then
+	 * this will assume that the faulting instruction was 32-bit.
+	 * If the faulting instruction was a 16-bit Thumb instruction,
+	 * then userspace would need to rewind the PC by 2 bytes prior to
+	 * resuming the vCPU (yuck!).
 	 */
 	kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
 
-- 
2.28.0.rc0.142.g3c755180ce-goog