On Tue, 23 Nov 2021 21:01:07 +0000,
Oliver Upton <oupton@xxxxxxxxxx> wrote:
>
> The OS lock blocks all debug exceptions at every EL. To date, KVM has
> not implemented the OS lock for its guests, despite the fact that it is
> mandatory per the architecture. Simple context switching between the
> guest and host is not appropriate, as its effects are not constrained to
> the guest context.
>
> Emulate the OS Lock by clearing MDE and SS in MDSCR_EL1, thereby
> blocking all but software breakpoint instructions. To handle breakpoint
> instructions, trap debug exceptions to EL2 and skip the instruction.
Skipping breakpoint instructions? I don't think you can do that, as
the guest does rely on BRK always being effective. I also don't see
where you do that...
>
> Signed-off-by: Oliver Upton <oupton@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/kvm_host.h | 4 ++++
> arch/arm64/kvm/debug.c | 27 +++++++++++++++++++++++----
> arch/arm64/kvm/sys_regs.c | 6 +++---
> 3 files changed, 30 insertions(+), 7 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 53fc8a6eaf1c..e5a06ff1cba6 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -726,6 +726,10 @@ void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu);
> 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);
> +
> +#define kvm_vcpu_os_lock_enabled(vcpu) \
> + (!!(__vcpu_sys_reg(vcpu, OSLSR_EL1) & SYS_OSLSR_OSLK))
> +
> int kvm_arm_vcpu_arch_set_attr(struct kvm_vcpu *vcpu,
> struct kvm_device_attr *attr);
> int kvm_arm_vcpu_arch_get_attr(struct kvm_vcpu *vcpu,
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index db9361338b2a..7835c76347ce 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -53,6 +53,14 @@ static void restore_guest_debug_regs(struct kvm_vcpu *vcpu)
> vcpu_read_sys_reg(vcpu, MDSCR_EL1));
> }
>
> +/*
> + * Returns true if the host needs to use the debug registers.
> + */
> +static inline bool host_using_debug_regs(struct kvm_vcpu *vcpu)
> +{
> + return vcpu->guest_debug || kvm_vcpu_os_lock_enabled(vcpu);
Just the name of the function has sent my head spinning. Even if the
*effects* of the host debug and the OS Lock are vaguely similar from
the guest PoV, they really are different things, and I'd rather not
lob them together.
> +}
> +
> /**
> * kvm_arm_init_debug - grab what we need for debug
> *
> @@ -105,9 +113,11 @@ static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu)
> * - Userspace is using the hardware to debug the guest
> * (KVM_GUESTDBG_USE_HW is set).
> * - The guest is not using debug (KVM_ARM64_DEBUG_DIRTY is clear).
> + * - The guest has enabled the OS Lock (debug exceptions are blocked).
> */
> if ((vcpu->guest_debug & KVM_GUESTDBG_USE_HW) ||
> - !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY))
> + !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY) ||
> + kvm_vcpu_os_lock_enabled(vcpu))
> vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>
> trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
> @@ -160,8 +170,10 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>
> kvm_arm_setup_mdcr_el2(vcpu);
>
> - /* Is Guest debugging in effect? */
> - if (vcpu->guest_debug) {
> + /*
> + * Check if we need to use the debug registers.
> + */
> + if (host_using_debug_regs(vcpu)) {
I'd rather you expand the helper here and add the comment you have in
the commit message explaining the machine-wide effect of the OS Lock.
> /* Save guest debug state */
> save_guest_debug_regs(vcpu);
>
> @@ -223,6 +235,10 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
> trace_kvm_arm_set_regset("WAPTS", get_num_wrps(),
> &vcpu->arch.debug_ptr->dbg_wcr[0],
> &vcpu->arch.debug_ptr->dbg_wvr[0]);
> + } else if (kvm_vcpu_os_lock_enabled(vcpu)) {
> + mdscr = vcpu_read_sys_reg(vcpu, MDSCR_EL1);
> + mdscr &= ~DBG_MDSCR_MDE;
> + vcpu_write_sys_reg(vcpu, mdscr, MDSCR_EL1);
> }
> }
>
> @@ -244,7 +260,10 @@ void kvm_arm_clear_debug(struct kvm_vcpu *vcpu)
> {
> trace_kvm_arm_clear_debug(vcpu->guest_debug);
>
> - if (vcpu->guest_debug) {
> + /*
> + * Restore the guest's debug registers if we were using them.
> + */
> + if (host_using_debug_regs(vcpu)) {
> restore_guest_debug_regs(vcpu);
>
> /*
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 5dbdb45d6d44..1346906f5c46 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1453,9 +1453,9 @@ static unsigned int mte_visibility(const struct kvm_vcpu *vcpu,
> * Debug handling: We do trap most, if not all debug related system
> * registers. The implementation is good enough to ensure that a guest
> * can use these with minimal performance degradation. The drawback is
> - * that we don't implement any of the external debug, none of the
> - * OSlock protocol. This should be revisited if we ever encounter a
> - * more demanding guest...
> + * that we don't implement any of the external debug architecture.
> + * This should be revisited if we ever encounter a more demanding
> + * guest...
> */
> static const struct sys_reg_desc sys_reg_descs[] = {
> { SYS_DESC(SYS_DC_ISW), access_dcsw },
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
_______________________________________________
kvmarm mailing list
kvmarm@xxxxxxxxxxxxxxxxxxxxx
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
