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Subject[RFC PATCH] kvm: arm64: handle single-step of hyp emulated mmio instructions
Date
There is a fast-path of MMIO emulation inside hyp mode. The handling
of single-step is broadly the same as kvm_arm_handle_step_debug()
except we just setup ESR/HSR so handle_exit() does the correct thing
as we exit.

For the case of an emulated illegal access causing an SError we signal
to handle_exit() by clearing the DBG_SPSR_SS bit as would have
actually happened had the hardware really single-stepped the
instruction.

[AJB: currently compile tested only]

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
arch/arm64/kvm/handle_exit.c | 8 +++++++-
arch/arm64/kvm/hyp/switch.c | 37 ++++++++++++++++++++++++++++++-------
2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index af1c804742f6..128120f04e0e 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -28,6 +28,7 @@
#include <asm/kvm_emulate.h>
#include <asm/kvm_mmu.h>
#include <asm/kvm_psci.h>
+#include <asm/debug-monitors.h>

#define CREATE_TRACE_POINTS
#include "trace.h"
@@ -242,7 +243,12 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
return 1;
case ARM_EXCEPTION_EL1_SERROR:
kvm_inject_vabt(vcpu);
- return 1;
+ /* We may still need to return for single-step */
+ if (!(*vcpu_cpsr(vcpu) & DBG_SPSR_SS)
+ && kvm_arm_handle_step_debug(vcpu, run))
+ return 0;
+ else
+ return 1;
case ARM_EXCEPTION_TRAP:
return handle_trap_exceptions(vcpu, run);
case ARM_EXCEPTION_HYP_GONE:
diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 945e79c641c4..a6712f179b52 100644
--- a/arch/arm64/kvm/hyp/switch.c
+++ b/arch/arm64/kvm/hyp/switch.c
@@ -22,6 +22,7 @@
#include <asm/kvm_emulate.h>
#include <asm/kvm_hyp.h>
#include <asm/fpsimd.h>
+#include <asm/debug-monitors.h>

static bool __hyp_text __fpsimd_enabled_nvhe(void)
{
@@ -263,7 +264,11 @@ static bool __hyp_text __populate_fault_info(struct kvm_vcpu *vcpu)
return true;
}

-static void __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
+/* Skip an instruction which has been emulated. Returns true if
+ * execution can continue or false if we need to exit hyp mode because
+ * single-step was in effect.
+ */
+static bool __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
{
*vcpu_pc(vcpu) = read_sysreg_el2(elr);

@@ -276,6 +281,14 @@ static void __hyp_text __skip_instr(struct kvm_vcpu *vcpu)
}

write_sysreg_el2(*vcpu_pc(vcpu), elr);
+
+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+ vcpu->arch.fault.esr_el2 =
+ (ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT) | 0x22;
+ return false;
+ } else {
+ return true;
+ }
}

int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
@@ -336,13 +349,21 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
int ret = __vgic_v2_perform_cpuif_access(vcpu);

if (ret == 1) {
- __skip_instr(vcpu);
- goto again;
+ if (__skip_instr(vcpu))
+ goto again;
+ else
+ exit_code = ARM_EXCEPTION_TRAP;
}

if (ret == -1) {
- /* Promote an illegal access to an SError */
- __skip_instr(vcpu);
+ /* Promote an illegal access to an
+ * SError. If we would be returning
+ * due to single-step clear the SS
+ * bit so handle_exit knows what to
+ * do after dealing with the error.
+ */
+ if (!__skip_instr(vcpu))
+ *vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
exit_code = ARM_EXCEPTION_EL1_SERROR;
}

@@ -357,8 +378,10 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
int ret = __vgic_v3_perform_cpuif_access(vcpu);

if (ret == 1) {
- __skip_instr(vcpu);
- goto again;
+ if (__skip_instr(vcpu))
+ goto again;
+ else
+ exit_code = ARM_EXCEPTION_TRAP;
}

/* 0 falls through to be handled out of EL2 */
--
2.15.0
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