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SubjectRe: [PATCH v1 1/2] KVM: arm64: handle single-stepping trapped instructions
On Fri, Oct 06, 2017 at 12:39:20PM +0100, Alex Bennée wrote:
> If we are using guest debug to single-step the guest we need to ensure
> we exit after emulating the instruction. This only affects
> instructions completely emulated by the kernel. For userspace emulated
> instructions we need to exit and return to complete the emulation.
>
> We fake debug.arch.hsr to contain ESR_ELx_EC_SOFTSTP_LOW so QEMU knows
> it was a single-step event (and without altering the userspace ABI).
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> ---
> arch/arm64/kvm/handle_exit.c | 48 +++++++++++++++++++++++++++++++-------------
> 1 file changed, 34 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 7debb74843a0..c918d291cb58 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -178,6 +178,39 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> return arm_exit_handlers[hsr_ec];
> }
>
> +/*
> + * When handling traps we need to ensure exit the guest if we
> + * completely emulated the instruction while single-stepping. Stuff to
> + * be emulated in userspace needs to complete that first.
> + */

I really don't understand the first sentence here. We are already out
of the guest, so do you mean a return to userspace?

I think the second sentence could be more clear as well. Is 'stuff' not
actually 'MMIO emulation' or 'emulation' more broadly?

> +
> +static int handle_trap_exceptions(struct kvm_vcpu *vcpu, struct kvm_run *run)
> +{
> + int handled;
> +
> + /*
> + * See ARM ARM B1.14.1: "Hyp traps on instructions
> + * that fail their condition code check"
> + */
> + if (!kvm_condition_valid(vcpu)) {
> + kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> + handled = 1;
> + } else {
> + exit_handle_fn exit_handler;
> +
> + exit_handler = kvm_get_exit_handler(vcpu);
> + handled = exit_handler(vcpu, run);
> + }
> +
> + if (handled && (vcpu->guest_debug & KVM_GUESTDBG_SINGLESTEP)) {

Don't you want if (handled == 1) or if (handled > 0) ?

If there was an error I think we want to just return that to userspace
and not override it and present single-stepping.

> + handled = 0;
> + run->exit_reason = KVM_EXIT_DEBUG;
> + run->debug.arch.hsr = ESR_ELx_EC_SOFTSTP_LOW << ESR_ELx_EC_SHIFT;
> + }
> +
> + return handled;
> +}
> +
> /*
> * Return > 0 to return to guest, < 0 on error, 0 (and set exit_reason) on
> * proper exit to userspace.
> @@ -185,8 +218,6 @@ static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
> int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
> int exception_index)
> {
> - exit_handle_fn exit_handler;
> -
> if (ARM_SERROR_PENDING(exception_index)) {
> u8 hsr_ec = ESR_ELx_EC(kvm_vcpu_get_hsr(vcpu));
>
> @@ -214,18 +245,7 @@ int handle_exit(struct kvm_vcpu *vcpu, struct kvm_run *run,
> kvm_inject_vabt(vcpu);
> return 1;
> case ARM_EXCEPTION_TRAP:
> - /*
> - * See ARM ARM B1.14.1: "Hyp traps on instructions
> - * that fail their condition code check"
> - */
> - if (!kvm_condition_valid(vcpu)) {
> - kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> - return 1;
> - }
> -
> - exit_handler = kvm_get_exit_handler(vcpu);
> -
> - return exit_handler(vcpu, run);
> + return handle_trap_exceptions(vcpu, run);
> case ARM_EXCEPTION_HYP_GONE:
> /*
> * EL2 has been reset to the hyp-stub. This happens when a guest
> --
> 2.14.1
>

Thanks,
-Christoffer

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