Re: [PATCH v2] KVM: arm64: Initialize VCPU mdcr_el2 before loading it

[Marc Zyngier <maz@kernel.org>]

Hi Alex,

On Tue, 23 Mar 2021 18:00:57 +0000,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> When a VCPU is created, the kvm_vcpu struct is initialized to zero in
> kvm_vm_ioctl_create_vcpu(). On VHE systems, the first time
> vcpu.arch.mdcr_el2 is loaded on hardware is in vcpu_load(), before it is
> set to a sensible value in kvm_arm_setup_debug() later in the run loop. The
> result is that KVM executes for a short time with MDCR_EL2 set to zero.
> 
> This has several unintended consequences:
> 
> * Setting MDCR_EL2.HPMN to 0 is constrained unpredictable according to ARM
>   DDI 0487G.a, page D13-3820. The behavior specified by the architecture
>   in this case is for the PE to behave as if MDCR_EL2.HPMN is set to a
>   value less than or equal to PMCR_EL0.N, which means that an unknown
>   number of counters are now disabled by MDCR_EL2.HPME, which is zero.
> 
> * The host configuration for the other debug features controlled by
>   MDCR_EL2 is temporarily lost. This has been harmless so far, as Linux
>   doesn't use the other fields, but that might change in the future.
> 
> Let's avoid both issues by initializing the VCPU's mdcr_el2 field in
> kvm_vcpu_vcpu_first_run_init(), thus making sure that the MDCR_EL2 register
> has a consistent value after each vcpu_load().
> 
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>

This looks strangely similar to 4942dc6638b0 ("KVM: arm64: Write
arch.mdcr_el2 changes since last vcpu_load on VHE"), just at a
different point. Probably worth a Fixes tag.

> ---
> Found by code inspection. Based on v5.12-rc4.
> 
> Tested on an odroid-c4 with VHE. vcpu->arch.mdcr_el2 is calculated to be
> 0x4e66. Without this patch, reading MDCR_EL2 after the first vcpu_load() in
> kvm_arch_vcpu_ioctl_run() returns 0; with this patch it returns the correct
> value, 0xe66 (FEAT_SPE is not implemented by the PE).
> 
> This patch was initially part of the KVM SPE series [1], but those patches
> haven't seen much activity, so I thought it would be a good idea to send
> this patch separately to draw more attention to it.
> 
> Changes in v2:
> * Moved kvm_arm_vcpu_init_debug() earlier in kvm_vcpu_first_run_init() so
>   vcpu->arch.mdcr_el2 is calculated even if kvm_vgic_map_resources() fails.
> * Added comment to kvm_arm_setup_mdcr_el2 to explain what testing
>   vcpu->guest_debug means.
> 
> [1] https://www.spinics.net/lists/kvm-arm/msg42959.html
> 
>  arch/arm64/include/asm/kvm_host.h |  1 +
>  arch/arm64/kvm/arm.c              |  3 +-
>  arch/arm64/kvm/debug.c            | 82 +++++++++++++++++++++----------
>  3 files changed, 59 insertions(+), 27 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 3d10e6527f7d..858c2fcfc043 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -713,6 +713,7 @@ static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>  static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>  
>  void kvm_arm_init_debug(void);
> +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);
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 7f06ba76698d..7088d8fe7186 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -580,6 +580,8 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>  
>  	vcpu->arch.has_run_once = true;
>  
> +	kvm_arm_vcpu_init_debug(vcpu);
> +
>  	if (likely(irqchip_in_kernel(kvm))) {
>  		/*
>  		 * Map the VGIC hardware resources before running a vcpu the
> @@ -791,7 +793,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		}
>  
>  		kvm_arm_setup_debug(vcpu);
> -

Spurious change?

>  		/**************************************************************
>  		 * Enter the guest
>  		 */
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 7a7e425616b5..3626d03354f6 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -68,6 +68,60 @@ void kvm_arm_init_debug(void)
>  	__this_cpu_write(mdcr_el2, kvm_call_hyp_ret(__kvm_get_mdcr_el2));
>  }
>  
> +/**
> + * kvm_arm_setup_mdcr_el2 - configure vcpu mdcr_el2 value
> + *
> + * @vcpu:	the vcpu pointer
> + * @host_mdcr:  host mdcr_el2 value
> + *
> + * This ensures we will trap access to:
> + *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> + *  - Debug ROM Address (MDCR_EL2_TDRA)
> + *  - OS related registers (MDCR_EL2_TDOSA)
> + *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> + */
> +static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu, u32 host_mdcr)
> +{
> +	bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
> +
> +	/*
> +	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
> +	 * to the profiling buffer.
> +	 */
> +	vcpu->arch.mdcr_el2 = host_mdcr & MDCR_EL2_HPMN_MASK;
> +	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> +				MDCR_EL2_TPMS |
> +				MDCR_EL2_TPMCR |
> +				MDCR_EL2_TDRA |
> +				MDCR_EL2_TDOSA);
> +
> +	/* Is the VM being debugged by userspace? */
> +	if (vcpu->guest_debug) {
> +		/* Route all software debug exceptions to EL2 */
> +		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> +		if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
> +			trap_debug = true;
> +	}
> +
> +	/* Trap debug register access */
> +	if (trap_debug)
> +		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> +
> +	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
> +}
> +
> +/**
> + * kvm_arm_vcpu_init_debug - setup vcpu debug traps
> + *
> + * @vcpu:	the vcpu pointer
> + *
> + * Set vcpu initial mdcr_el2 value.
> + */
> +void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu)
> +{
> +	kvm_arm_setup_mdcr_el2(vcpu, this_cpu_read(mdcr_el2));

Given that kvm_arm_setup_mdcr_el2() always takes the current host
value for mdcr_el2, why not moving the read into it and be done with
it?

Also, do we really need an extra wrapper?

> +}
> +
>  /**
>   * kvm_arm_reset_debug_ptr - reset the debug ptr to point to the vcpu state
>   */
> @@ -83,12 +137,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>   * @vcpu:	the vcpu pointer
>   *
>   * This is called before each entry into the hypervisor to setup any
> - * debug related registers. Currently this just ensures we will trap
> - * access to:
> - *  - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
> - *  - Debug ROM Address (MDCR_EL2_TDRA)
> - *  - OS related registers (MDCR_EL2_TDOSA)
> - *  - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
> + * debug related registers.
>   *
>   * Additionally, KVM only traps guest accesses to the debug registers if
>   * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
> @@ -100,27 +149,14 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>  
>  void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  {
> -	bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
>  	unsigned long mdscr, orig_mdcr_el2 = vcpu->arch.mdcr_el2;
>  
>  	trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
>  
> -	/*
> -	 * This also clears MDCR_EL2_E2PB_MASK to disable guest access
> -	 * to the profiling buffer.
> -	 */
> -	vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
> -	vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
> -				MDCR_EL2_TPMS |
> -				MDCR_EL2_TPMCR |
> -				MDCR_EL2_TDRA |
> -				MDCR_EL2_TDOSA);
> +	kvm_arm_setup_mdcr_el2(vcpu, __this_cpu_read(mdcr_el2));
>  
>  	/* Is Guest debugging in effect? */
>  	if (vcpu->guest_debug) {
> -		/* Route all software debug exceptions to EL2 */
> -		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
> -
>  		/* Save guest debug state */
>  		save_guest_debug_regs(vcpu);
>  
> @@ -174,7 +210,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  
>  			vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
>  			vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
> -			trap_debug = true;

There is something that slightly worries me here: there is now a
disconnect between flagging debug as dirty and setting the
trapping. And actually, you now check for KVM_ARM64_DEBUG_DIRTY and
set the trap bits *before* setting the dirty bit itself.

Here, I believe you end up with guest/host confusion of breakpoints,
which isn't great. Or did I miss something?

>  
>  			trace_kvm_arm_set_regset("BKPTS", get_num_brps(),
>  						&vcpu->arch.debug_ptr->dbg_bcr[0],
> @@ -189,10 +224,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  	BUG_ON(!vcpu->guest_debug &&
>  		vcpu->arch.debug_ptr != &vcpu->arch.vcpu_debug_state);
>  
> -	/* Trap debug register access */
> -	if (trap_debug)
> -		vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
> -
>  	/* If KDE or MDE are set, perform a full save/restore cycle. */
>  	if (vcpu_read_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))
>  		vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
> @@ -201,7 +232,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>  	if (has_vhe() && orig_mdcr_el2 != vcpu->arch.mdcr_el2)
>  		write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
>  
> -	trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
>  	trace_kvm_arm_set_dreg32("MDSCR_EL1", vcpu_read_sys_reg(vcpu, MDSCR_EL1));
>  }

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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