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

["Alex Bennée" <alex.bennee@linaro.org>]

Christoffer Dall <christoffer.dall@linaro.org> writes:

> 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@linaro.org>
>> 
>>  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@linaro.org>
>> + *
>> + * 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.

Yeah because I couldn't see it being set I figured it was OK to blat it
with zeros. But you are right we should set it up properly if we are
going to mess with it.

>
> Thanks,
> -Christoffer

-- 
Alex Bennée