Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear_debug()

From: Christoffer Dall
Date: Mon Apr 13 2015 - 10:36:24 EST

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On Tue, Mar 31, 2015 at 04:08:03PM +0100, Alex Bennée wrote:
> This is a precursor for later patches which will need to do more to
> setup debug state before entering the hyp.S switch code. The existing
> functionality for setting mdcr_el2 has been moved out of hyp.S and now
> uses the value kept in vcpu->arch.mdcr_el2.
>
> This also moves the conditional setting of the TDA bit from the hyp code
> into the C code.
>
> Signed-off-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
>
> create mode 100644 arch/arm64/kvm/debug.c
>
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 41008cd..8c01c97 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -242,5 +242,7 @@ static inline void kvm_arch_hardware_unsetup(void) {}
> static inline void kvm_arch_sync_events(struct kvm *kvm) {}
> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
> +static inline void kvm_arch_setup_debug(struct kvm_vcpu *vcpu) {}
> +static inline void kvm_arch_clear_debug(struct kvm_vcpu *vcpu) {}
>
> #endif /* __ARM_KVM_HOST_H__ */
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 445933d..7ea8b0e 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -523,6 +523,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
>
> kvm_vgic_flush_hwstate(vcpu);
> kvm_timer_flush_hwstate(vcpu);
> + kvm_arch_setup_debug(vcpu);
>
> local_irq_disable();
>
> @@ -569,6 +570,7 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *run)
> * Back from guest
> *************************************************************/
>
> + kvm_arch_clear_debug(vcpu);
> kvm_timer_sync_hwstate(vcpu);
> kvm_vgic_sync_hwstate(vcpu);
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 8ac3c70..0631840 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -101,6 +101,7 @@ struct kvm_vcpu_arch {
>
> /* HYP configuration */
> u64 hcr_el2;
> + u32 mdcr_el2;
>
> /* Exception Information */
> struct kvm_vcpu_fault_info fault;
> @@ -257,4 +258,7 @@ static inline void kvm_arch_sync_events(struct kvm *kvm) {}
> static inline void kvm_arch_vcpu_uninit(struct kvm_vcpu *vcpu) {}
> static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>
> +void kvm_arch_setup_debug(struct kvm_vcpu *vcpu);
> +void kvm_arch_clear_debug(struct kvm_vcpu *vcpu);
> +
> #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
> index f7fa65d..cd06209 100644
> --- a/arch/arm64/kernel/asm-offsets.c
> +++ b/arch/arm64/kernel/asm-offsets.c
> @@ -122,6 +122,7 @@ int main(void)
> DEFINE(VCPU_HPFAR_EL2, offsetof(struct kvm_vcpu, arch.fault.hpfar_el2));
> DEFINE(VCPU_DEBUG_FLAGS, offsetof(struct kvm_vcpu, arch.debug_flags));
> DEFINE(VCPU_HCR_EL2, offsetof(struct kvm_vcpu, arch.hcr_el2));
> + DEFINE(VCPU_MDCR_EL2, offsetof(struct kvm_vcpu, arch.mdcr_el2));
> DEFINE(VCPU_IRQ_LINES, offsetof(struct kvm_vcpu, arch.irq_lines));
> DEFINE(VCPU_HOST_CONTEXT, offsetof(struct kvm_vcpu, arch.host_cpu_context));
> DEFINE(VCPU_TIMER_CNTV_CTL, offsetof(struct kvm_vcpu, arch.timer_cpu.cntv_ctl));
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index 4e6e09e..6796d4a 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -17,7 +17,7 @@ kvm-$(CONFIG_KVM_ARM_HOST) += $(ARM)/psci.o $(ARM)/perf.o
>
> kvm-$(CONFIG_KVM_ARM_HOST) += emulate.o inject_fault.o regmap.o
> kvm-$(CONFIG_KVM_ARM_HOST) += hyp.o hyp-init.o handle_exit.o
> -kvm-$(CONFIG_KVM_ARM_HOST) += guest.o reset.o sys_regs.o sys_regs_generic_v8.o
> +kvm-$(CONFIG_KVM_ARM_HOST) += guest.o debug.o reset.o sys_regs.o sys_regs_generic_v8.o
>
> kvm-$(CONFIG_KVM_ARM_VGIC) += $(KVM)/arm/vgic.o
> kvm-$(CONFIG_KVM_ARM_VGIC) += $(KVM)/arm/vgic-v2.o
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> new file mode 100644
> index 0000000..8a29d0b
> --- /dev/null
> +++ b/arch/arm64/kvm/debug.c
> @@ -0,0 +1,58 @@
> +/*
> + * Debug and Guest Debug support
> + *
> + * Copyright (C) 2015 - Linaro Ltd
> + * Author: Alex Bennée <alex.bennee@xxxxxxxxxx>
> + *
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License version 2 as
> + * published by the Free Software Foundation.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program. If not, see <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <linux/kvm_host.h>
> +
> +#include <asm/kvm_arm.h>
> +#include <asm/kvm_host.h>
> +
> +/**
> + * kvm_arch_setup_debug - set-up debug related stuff

nit: I think you want "set up" when it's a verb.

> + *
> + * @vcpu: the vcpu pointer
> + *
> + * This is called before each entry in to the hypervisor to setup any

s/in to/into/
s/setup/set up/

> + * debug related registers. Currently this just ensures we will trap
> + * access to:

guest accesses to:

> + * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> + * - Debug ROM Address (MDCR_EL2_TDRA)
> + * - Power down debug registers (MDCR_EL2_TDOSA)
> + *
> + * Additionally the hypervisor lazily saves/restores the debug
> + * register state. If it is not currently doing so (arch.debug_flags)
> + * then we also need to ensure we trap if the guest messes with them
> + * so we know we need to save them.

This paragraph is a little hard to make sense of. If I understand it
correctly, the point is that when debugging the guest we need to make
sure guest accesses to the debug registers traps? If so, I would
suggest something like:

Additionally, KVM only traps guest accesses to the debug registers if
the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
flag on vcpu->arch.debug_flags). Since the guest must not interfere
with the hardware state when debugging the guest, we must ensure that
trapping is enabled whenever we are debugging the guest.

> + */
> +
> +void kvm_arch_setup_debug(struct kvm_vcpu *vcpu)
> +{
> + vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM | MDCR_EL2_TPMCR);
> + vcpu->arch.mdcr_el2 |= (MDCR_EL2_TDRA | MDCR_EL2_TDOSA);
> +
> + if (!vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
> + vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> + else
> + vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDA;
> +
> +}
> +
> +void kvm_arch_clear_debug(struct kvm_vcpu *vcpu)
> +{
> + /* Nothing to do yet */
> +}
> diff --git a/arch/arm64/kvm/hyp.S b/arch/arm64/kvm/hyp.S
> index 5befd01..be92bfe1 100644
> --- a/arch/arm64/kvm/hyp.S
> +++ b/arch/arm64/kvm/hyp.S
> @@ -768,17 +768,8 @@
> mov x2, #(1 << 15) // Trap CP15 Cr=15
> msr hstr_el2, x2
>
> - mrs x2, mdcr_el2
> - and x2, x2, #MDCR_EL2_HPMN_MASK
> - orr x2, x2, #(MDCR_EL2_TPM | MDCR_EL2_TPMCR)
> - orr x2, x2, #(MDCR_EL2_TDRA | MDCR_EL2_TDOSA)
> -
> - // Check for KVM_ARM64_DEBUG_DIRTY, and set debug to trap
> - // if not dirty.
> - ldr x3, [x0, #VCPU_DEBUG_FLAGS]
> - tbnz x3, #KVM_ARM64_DEBUG_DIRTY_SHIFT, 1f
> - orr x2, x2, #MDCR_EL2_TDA
> -1:
> + // Monitor Debug Config - see kvm_arch_setup_debug()
> + ldr x2, [x0, #VCPU_MDCR_EL2]
> msr mdcr_el2, x2
> .endm
>

As the other reviewers noted, this is now setting the number of PMU
counters accessible to 0. I'm fine with always setting mdcr_el2 from
memory, but I think we need to add code in hyp-init.S to read PMCR_EL0.N
and store that in a per-cpu (not vcpu) variable and configure mdcr_el2
using that value in kvm_arch_set_debug(), but then you need to call
kvm_arch_set_debug() from a non-preemptible section, which is probably a
good idea anyway. Note that I couldn't see that we are initializing
MDCR_EL2 anywhere, so that should probably be a separate fix.

Thanks,
-Christoffer
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Next message: Arnaldo Carvalho de Melo: "Re: [PATCH 1/3] perf probe: Set retprobe flag when probe in address-based alternative mode"
Previous message: Michal Simek: "[PATCH] serial: of-serial: Remove device_type = "serial" registration"
In reply to: Alex BennÃe: "Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear_debug()"
Next in thread: Christoffer Dall: "Re: [PATCH v2 05/10] KVM: arm: introduce kvm_arch_setup/clear_debug()"
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