arm64: KVM: Handle async aborts delivered while at EL2

If EL1 generates an asynchronous abort and then traps into EL2
before the abort has been delivered, we may end-up with the
abort firing at the worse possible place: on the host.

In order to avoid this, it is necessary to take the abort at EL2,
by clearing the PSTATE.A bit. In order to survive this abort,
we do it at a point where we're in a known state with respect
to the world switch, and handle the resulting exception,
overloading the exit code in the process.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>

diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
index b5926ee..12ee62d 100644 (file)
--- a/arch/arm64/kvm/hyp/entry.S
+++ b/arch/arm64/kvm/hyp/entry.S
@@ -124,7 +124,38 @@ ENTRY(__guest_exit)
        // Now restore the host regs
        restore_callee_saved_regs x2
 
-       ret
+       // If we have a pending asynchronous abort, now is the
+       // time to find out. From your VAXorcist book, page 666:
+       // "Threaten me not, oh Evil one!  For I speak with
+       // the power of DEC, and I command thee to show thyself!"
+       mrs     x2, elr_el2
+       mrs     x3, esr_el2
+       mrs     x4, spsr_el2
+       mov     x5, x0
+
+       dsb     sy              // Synchronize against in-flight ld/st
+       msr     daifclr, #4     // Unmask aborts
+
+       // This is our single instruction exception window. A pending
+       // SError is guaranteed to occur at the earliest when we unmask
+       // it, and at the latest just after the ISB.
+       .global abort_guest_exit_start
+abort_guest_exit_start:
+
+       isb
+
+       .global abort_guest_exit_end
+abort_guest_exit_end:
+
+       // If the exception took place, restore the EL1 exception
+       // context so that we can report some information.
+       // Merge the exception code with the SError pending bit.
+       tbz     x0, #ARM_EXIT_WITH_SERROR_BIT, 1f
+       msr     elr_el2, x2
+       msr     esr_el2, x3
+       msr     spsr_el2, x4
+       orr     x0, x0, x5
+1:     ret
 ENDPROC(__guest_exit)
 
 ENTRY(__fpsimd_guest_restore)

diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
index d2f6640..4e92399 100644 (file)
--- a/arch/arm64/kvm/hyp/hyp-entry.S
+++ b/arch/arm64/kvm/hyp/hyp-entry.S
@@ -126,6 +126,28 @@ el1_error:
        mov     x0, #ARM_EXCEPTION_EL1_SERROR
        b       __guest_exit
 
+el2_error:
+       /*
+        * Only two possibilities:
+        * 1) Either we come from the exit path, having just unmasked
+        *    PSTATE.A: change the return code to an EL2 fault, and
+        *    carry on, as we're already in a sane state to handle it.
+        * 2) Or we come from anywhere else, and that's a bug: we panic.
+        *
+        * For (1), x0 contains the original return code and x1 doesn't
+        * contain anything meaningful at that stage. We can reuse them
+        * as temp registers.
+        * For (2), who cares?
+        */
+       mrs     x0, elr_el2
+       adr     x1, abort_guest_exit_start
+       cmp     x0, x1
+       adr     x1, abort_guest_exit_end
+       ccmp    x0, x1, #4, ne
+       b.ne    __hyp_panic
+       mov     x0, #(1 << ARM_EXIT_WITH_SERROR_BIT)
+       eret
+
 ENTRY(__hyp_do_panic)
        mov     lr, #(PSR_F_BIT | PSR_I_BIT | PSR_A_BIT | PSR_D_BIT |\
                      PSR_MODE_EL1h)
@@ -150,7 +172,6 @@ ENDPROC(\label)
        invalid_vector  el2h_sync_invalid
        invalid_vector  el2h_irq_invalid
        invalid_vector  el2h_fiq_invalid
-       invalid_vector  el2h_error_invalid
        invalid_vector  el1_sync_invalid
        invalid_vector  el1_irq_invalid
        invalid_vector  el1_fiq_invalid
@@ -168,7 +189,7 @@ ENTRY(__kvm_hyp_vector)
        ventry  el2h_sync_invalid               // Synchronous EL2h
        ventry  el2h_irq_invalid                // IRQ EL2h
        ventry  el2h_fiq_invalid                // FIQ EL2h
-       ventry  el2h_error_invalid              // Error EL2h
+       ventry  el2_error                       // Error EL2h
 
        ventry  el1_sync                        // Synchronous 64-bit EL1
        ventry  el1_irq                         // IRQ 64-bit EL1

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 8246de2..8b81cc6 100644 (file)
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -292,6 +292,12 @@ again:
        exit_code = __guest_enter(vcpu, host_ctxt);
        /* And we're baaack! */
 
+       /*
+        * We're using the raw exception code in order to only process
+        * the trap if no SError is pending. We will come back to the
+        * same PC once the SError has been injected, and replay the
+        * trapping instruction.
+        */
        if (exit_code == ARM_EXCEPTION_TRAP && !__populate_fault_info(vcpu))
                goto again;