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Subject[PATCH v5 07/12] KVM: arm64: guest debug, add support for single-step
Date
This adds support for single-stepping the guest. To do this we need to
manipulate the guests PSTATE.SS and MDSCR_EL1.SS bits which we do in the
kvm_arm_setup/clear_debug() so we don't affect the apparent state of the
guest. Additionally while the host is debugging the guest we suppress
the ability of the guest to single-step itself.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

---
v2
- Move pstate/mdscr manipulation into C
- don't export guest_debug to assembly
- add accessor for saved_debug regs
- tweak save/restore of mdscr_el1
v3
- don't save PC in debug information struct
- rename debug_saved_regs->guest_debug_state
- save whole value, only use bits in restore
- add save/restore_guest-debug_regs helper functions
- simplify commit message for clarity
- rm vcpu_debug_saved_reg access fn
v4
- added more comments based on suggestions
- guest_debug_state->guest_debug_preserved
- no point masking restore, we will trap out
v5
- more comments
- don't bother preserving pstate.ss
---
arch/arm/kvm/arm.c | 4 ++-
arch/arm64/include/asm/kvm_host.h | 11 ++++++++
arch/arm64/kvm/debug.c | 58 ++++++++++++++++++++++++++++++++++++---
arch/arm64/kvm/handle_exit.c | 2 ++
4 files changed, 70 insertions(+), 5 deletions(-)

diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
index 064c105..9b3ed6d 100644
--- a/arch/arm/kvm/arm.c
+++ b/arch/arm/kvm/arm.c
@@ -302,7 +302,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
kvm_arm_set_running_vcpu(NULL);
}

-#define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE | KVM_GUESTDBG_USE_SW_BP)
+#define KVM_GUESTDBG_VALID_MASK (KVM_GUESTDBG_ENABLE | \
+ KVM_GUESTDBG_USE_SW_BP | \
+ KVM_GUESTDBG_SINGLESTEP)

/**
* kvm_arch_vcpu_ioctl_set_guest_debug - set up guest debugging
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 7cb99b5..e2db6a6 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -123,6 +123,17 @@ struct kvm_vcpu_arch {
* here.
*/

+ /*
+ * Guest registers we preserve during guest debugging.
+ *
+ * These shadow registers are updated by the kvm_handle_sys_reg
+ * trap handler if the guest accesses or updates them while we
+ * are using guest debug.
+ */
+ struct {
+ u32 mdscr_el1;
+ } guest_debug_preserved;
+
/* Don't run the guest */
bool pause;

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index 8d1bfa4..10a6baa 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c
@@ -19,11 +19,41 @@

#include <linux/kvm_host.h>

+#include <asm/debug-monitors.h>
+#include <asm/kvm_asm.h>
#include <asm/kvm_arm.h>
+#include <asm/kvm_emulate.h>
+
+/* These are the bits of MDSCR_EL1 we may manipulate */
+#define MDSCR_EL1_DEBUG_MASK (DBG_MDSCR_SS | \
+ DBG_MDSCR_KDE | \
+ DBG_MDSCR_MDE)

static DEFINE_PER_CPU(u32, mdcr_el2);

/**
+ * save/restore_guest_debug_regs
+ *
+ * For some debug operations we need to tweak some guest registers. As
+ * a result we need to save the state of those registers before we
+ * make those modifications. This does get confused if the guest
+ * attempts to control single step while being debugged. It will start
+ * working again once it is no longer being debugged by the host.
+ *
+ * Guest access to MDSCR_EL1 is trapped by the hypervisor and handled
+ * after we have restored the preserved value to the main context.
+ */
+static void save_guest_debug_regs(struct kvm_vcpu *vcpu)
+{
+ vcpu->arch.guest_debug_preserved.mdscr_el1 = vcpu_sys_reg(vcpu, MDSCR_EL1);
+}
+
+static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
+{
+ vcpu_sys_reg(vcpu, MDSCR_EL1) = vcpu->arch.guest_debug_preserved.mdscr_el1;
+}
+
+/**
* kvm_arm_init_debug - grab what we need for debug
*
* Currently the sole task of this function is to retrieve the initial
@@ -38,7 +68,6 @@ void kvm_arm_init_debug(void)
__this_cpu_write(mdcr_el2, kvm_call_hyp(__kvm_get_mdcr_el2));
}

-
/**
* kvm_arm_setup_debug - set up debug related stuff
*
@@ -73,12 +102,33 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
if (trap_debug)
vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;

- /* Trap breakpoints? */
- if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
+ /* Is Guest debugging in effect? */
+ if (vcpu->guest_debug) {
vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
+
+ /* Save guest debug state */
+ save_guest_debug_regs(vcpu);
+
+ /*
+ * Single Step (ARM ARM D2.12.3 The software step state
+ * machine)
+ *
+ * If we are doing Single Step we need to manipulate
+ * MDSCR_EL1.SS and PSTATE.SS. If not we need to
+ * suppress the guests ability to trigger single step
+ * exceptions as otherwise the host will deal with them.
+ */
+ if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
+ *vcpu_cpsr(vcpu) |= DBG_SPSR_SS;
+ vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_SS;
+ } else {
+ vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
+ }
+ }
}

void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
{
- /* Nothing to do yet */
+ if (vcpu->guest_debug)
+ restore_guest_debug_regs(vcpu);
}
diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
index 27f38a9..e9de13e 100644
--- a/arch/arm64/kvm/handle_exit.c
+++ b/arch/arm64/kvm/handle_exit.c
@@ -103,6 +103,7 @@ static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
run->debug.arch.hsr = hsr;

switch (hsr >> ESR_ELx_EC_SHIFT) {
+ case ESR_ELx_EC_SOFTSTP_LOW:
case ESR_ELx_EC_BKPT32:
case ESR_ELx_EC_BRK64:
break;
@@ -130,6 +131,7 @@ static exit_handle_fn arm_exit_handlers[] = {
[ESR_ELx_EC_SYS64] = kvm_handle_sys_reg,
[ESR_ELx_EC_IABT_LOW] = kvm_handle_guest_abort,
[ESR_ELx_EC_DABT_LOW] = kvm_handle_guest_abort,
+ [ESR_ELx_EC_SOFTSTP_LOW]= kvm_handle_guest_debug,
[ESR_ELx_EC_BKPT32] = kvm_handle_guest_debug,
[ESR_ELx_EC_BRK64] = kvm_handle_guest_debug,
};
--
2.4.1


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