On 31/08/17 09:54, Christoffer Dall wrote:

On Thu, Aug 31, 2017 at 10:45 AM, Julien Thierry [off-list ref] wrote:

quoted

Hi Christoffer,


On 30/08/17 19:53, Christoffer Dall wrote:

quoted

Hi Julien,

[cc'ing Alex Benn?e here who wrote the debug code for arm64]

On Wed, Aug 30, 2017 at 11:01 AM, Julien Thierry [off-list ref]
wrote:

quoted

Software Step exception is missing after trapping instruction from the
guest.

We need to set the PSR.SS to 0 for the guest vcpu before resuming guest
execution.

Signed-off-by: Julien Thierry <redacted>
Cc: Christoffer Dall <redacted>
Cc: Marc Zyngier <redacted>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <redacted>

---
   arch/arm64/include/asm/kvm_asm.h     |  2 ++
   arch/arm64/include/asm/kvm_emulate.h |  2 ++
   arch/arm64/kvm/debug.c               | 12 +++++++++++-
   arch/arm64/kvm/hyp/switch.c          | 12 ++++++++++++
   4 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h

b/arch/arm64/include/asm/kvm_asm.h
index 26a64d0..398bbaa 100644

--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h

@@ -32,6 +32,8 @@

   #define KVM_ARM64_DEBUG_DIRTY_SHIFT    0
   #define KVM_ARM64_DEBUG_DIRTY          (1 <<

KVM_ARM64_DEBUG_DIRTY_SHIFT)
+#define KVM_ARM64_DEBUG_INST_SKIP_SHIFT        1
+#define KVM_ARM64_DEBUG_INST_SKIP      (1 <<
KVM_ARM64_DEBUG_INST_SKIP_SHIFT)

   #define kvm_ksym_ref(sym)
\
          ({
\

diff --git a/arch/arm64/include/asm/kvm_emulate.h

b/arch/arm64/include/asm/kvm_emulate.h
index fe39e68..d401c64 100644

--- a/arch/arm64/include/asm/kvm_emulate.h
+++ b/arch/arm64/include/asm/kvm_emulate.h

@@ -95,6 +95,8 @@ static inline void kvm_skip_instr(struct kvm_vcpu

*vcpu, bool is_wide_instr)
                  kvm_skip_instr32(vcpu, is_wide_instr);
          else
                  *vcpu_pc(vcpu) += 4;
+       /* Let debug engine know we skipped an instruction */
+       vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_INST_SKIP;

Why do we need to defer this action until later?

Can't we simply do clear DBG_SPSR_SS here?

That was my first intention, but it turns out that the current state of
things (without this patch) is that every time we enter a guest,
kvm_arm_setup_debug gets called and if single step is requested for the
guest it will set the flag in the SPSR (ignoring the fact that we cleared
it).

Ah, right, duh.

quoted

This happens even if we exit the guest because of a data abort.

For normal single step execution, we do need to reset SPSR.SS to 1 before
running the guest since completion of a step should clear that bit before
taking a software step exception. So what kvm_arm_setup_debug does seems
correct to me but missed the case for trapped/emulated instructions.

So even if we just clear DBG_SPSR_SS here, we would still need to tell
kvm_arm_setup_debug not to change the bit. Or resetting SPSR.SS to 1 for
normal single stepping needs to be done before we skip instructions in KVM
but that doesn't sound right to me...

So I'm wondering if we're going about this wrong.  Perhaps we need to
discover at the end of the run loop that we were asked to single step
execution and simply return to userspace, setting the debug exit
reason etc., instead of entering the guest with PSTATE.SS==0 and
relying on another trap back in to the guest just to set two fields on
the kvm_run structure and exit to user space ?

So if I understand correctly, the suggestion is that when we trap an 
instruction we check whether it was supposed to be single stepped, if it 
was we set up the vcpu registers as if it had taken a software step 
exception and return from kvm_arch_vcpu_ioctl_run. Is that right?

Interesting idea, I can try to explore that possibility.

Thanks for the suggestion,

quoted

quoted

I think even if the guest kernel is single-stepping guest userspace,
you're still safe because you'll take another debug exception from
having 'executed' (read: emulated) this instruction in the hypervisor.

quoted

   }

   static inline void vcpu_set_thumb(struct kvm_vcpu *vcpu)

diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
index dbadfaf..4806e6b 100644
--- a/arch/arm64/kvm/debug.c
+++ b/arch/arm64/kvm/debug.c

@@ -151,12 +151,22 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
                   * debugging the system.
                   */
                  if (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP) {
-                       *vcpu_cpsr(vcpu) |=  DBG_SPSR_SS;
+                       /*
+                        * If we skipped an instruction while single

stepping,
+                        * spsr.ss needs to be 0 in order to trigger SS
+                        * exception on return.
+                        */
+                       if (!(vcpu->arch.debug_flags &
KVM_ARM64_DEBUG_INST_SKIP))
+                               *vcpu_cpsr(vcpu) |=  DBG_SPSR_SS;
+                       else
+                               *vcpu_cpsr(vcpu) &=  ~DBG_SPSR_SS;

Then you wouldn't need this.

quoted

                          vcpu_sys_reg(vcpu, MDSCR_EL1) |= DBG_MDSCR_SS;
                  } else {
                          vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~DBG_MDSCR_SS;
                  }

+               vcpu->arch.debug_flags &= ~KVM_ARM64_DEBUG_INST_SKIP;
+
                  trace_kvm_arm_set_dreg32("SPSR_EL2", *vcpu_cpsr(vcpu));

                  /*

diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
index 945e79c..6030acd 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)
   {

@@ -276,6 +277,10 @@ static void __hyp_text __skip_instr(struct kvm_vcpu

*vcpu)
          }

          write_sysreg_el2(*vcpu_pc(vcpu), elr);
+
+       *vcpu_cpsr(vcpu) = read_sysreg_el2(spsr);
+       *vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
+       write_sysreg_el2(*vcpu_cpsr(vcpu), spsr);

I don't think you really need to use *vcpu_cpsr(vcpu) here, I think
you can just use a temporary variable, which may make the flow a bit
easier to read actually.

Good point, I'll do that.

quoted

quoted

   }

   int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)

@@ -343,6 +348,13 @@ int __hyp_text __kvm_vcpu_run(struct kvm_vcpu *vcpu)
                          if (ret == -1) {
                                  /* Promote an illegal access to an

SError */
                                  __skip_instr(vcpu);
+
+                               /*
+                                * We're not jumping back, let debug
setup know
+                                * we skipped an instruction.
+                                */
+                               vcpu->arch.debug_flags |=
KVM_ARM64_DEBUG_INST_SKIP;

I agree with Marc, I don't think you need this, because you have
modified the hardware spsr_el2, which you're going to read back in
__sysreg_save_guest_state().

We are exiting the guest, so it is the same case as kvm_skip_instr.

ok, I see this now, you're right.  Thanks for explaining it.  For some
reason, this code is just really hard to understand, so I wonder if
the suggested approach above is better.

Thanks,
-Christoffer

-- 
Julien Thierry