Re: [PATCH v1 2/2] kvm: arm64: handle single-step of userspace mmio instructions

From: Julien Thierry
Date: Fri Oct 06 2017 - 10:27:58 EST

On 06/10/17 14:45, Alex BennÃe wrote:

Julien Thierry <julien.thierry@xxxxxxx> writes:

On 06/10/17 12:39, Alex BennÃe wrote:
The system state of KVM when using userspace emulation is not complete
until we return into KVM_RUN. To handle mmio related updates we wait
until they have been committed and then schedule our KVM_EXIT_DEBUG.

I've introduced a new function kvm_arm_maybe_return_debug() to wrap up
the differences between arm/arm64 which is currently null for arm.

Signed-off-by: Alex BennÃe <alex.bennee@xxxxxxxxxx>
arch/arm/include/asm/kvm_host.h | 2 ++
arch/arm64/include/asm/kvm_host.h | 1 +
arch/arm64/kvm/debug.c | 21 +++++++++++++++++++++
arch/arm64/kvm/handle_exit.c | 9 +++------
virt/kvm/arm/arm.c | 2 +-
virt/kvm/arm/mmio.c | 3 ++-
6 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
index 4a879f6ff13b..aec943f6d123 100644
--- a/arch/arm/include/asm/kvm_host.h
+++ b/arch/arm/include/asm/kvm_host.h
@@ -285,6 +285,8 @@ static inline void kvm_arm_init_debug(void) {}
static inline void kvm_arm_setup_debug(struct kvm_vcpu *vcpu) {}
static inline void kvm_arm_clear_debug(struct kvm_vcpu *vcpu) {}
static inline void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu) {}
+static inline int kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu,
+ struct kvm_run *run) {}

This function should return 1.

So I did ponder making this a bool, returning true if we need to exit
and testing in v/k/a/arm.c exit leg rather than in the mmio handler.

At the moment it mirrors the existing exit logic which follows -1 err, 0
return, >0 handled. But as I mentioned in the cover letter this fell
down a bit when dealing with the mmio case.

Hmmm, my main issue is that this version doesn't have a return statement, which probably triggers a gcc warning with ARCH=arm and also might cause arm (32bit) kvm to exit upon handling mmio return when we don't want to.

Otherwise, I also wondered about using a bool here. But following the pre-existing logic makes sense to me (but I have no strong feeling about it).

int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
struct kvm_device_attr *attr);
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index e923b58606e2..fa67d21662f6 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -369,6 +369,7 @@ void kvm_arm_init_debug(void);
void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
+int kvm_arm_maybe_return_debug(struct kvm_vcpu *vcpu, struct kvm_run *run);

I feel the name could be a little bit more explicit:

kvm_arm_trap_need_step_debug, kvm_arm_trap_step_return_debug,

I wanted to keep the debug suffix so that's fine although I'm not so
sure trap is correct because on the tail end of mmio emulation are we
still trapping?

Maybe kvm_arm_step_emulated_debug?

Yes, sounds good.


At least, I think it would be nice that the name reflect that this
check is meant for emulated instructions.


Reviewed-by: Julien Thierry <julien.thierry@xxxxxxx>


Alex BennÃe

Julien Thierry