Subject: Re: [PATCH v4 06/15] KVM: arm64: Restore mdcr_el2 from vcpu

From: Marc Zyngier <[email-masked]>
Date: Wed, 18 Aug 2021 15:42:23 +0100

On Tue, 17 Aug 2021 09:11:25 +0100,
Fuad Tabba <[email-masked]> wrote:
> 
> On deactivating traps, restore the value of mdcr_el2 from the
> newly created and preserved host value vcpu context, rather than
> directly reading the hardware register.
> 
> Up until and including this patch the two values are the same,
> i.e., the hardware register and the vcpu one. A future patch will
> be changing the value of mdcr_el2 on activating traps, and this
> ensures that its value will be restored.
> 
> No functional change intended.
> 
> Signed-off-by: Fuad Tabba <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_host.h       |  5 ++++-
>  arch/arm64/include/asm/kvm_hyp.h        |  2 +-
>  arch/arm64/kvm/hyp/include/hyp/switch.h |  6 +++++-
>  arch/arm64/kvm/hyp/nvhe/switch.c        | 13 +++++--------
>  arch/arm64/kvm/hyp/vhe/switch.c         | 14 +++++---------
>  arch/arm64/kvm/hyp/vhe/sysreg-sr.c      |  2 +-
>  6 files changed, 21 insertions(+), 21 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 4d2d974c1522..76462c6a91ee 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -287,10 +287,13 @@ struct kvm_vcpu_arch {
>  	/* Stage 2 paging state used by the hardware on next switch */
>  	struct kvm_s2_mmu *hw_mmu;
>  
> -	/* HYP configuration */
> +	/* Values of trap registers for the guest. */
>  	u64 hcr_el2;
>  	u64 mdcr_el2;
>  
> +	/* Values of trap registers for the host before guest entry. */
> +	u64 mdcr_el2_host;

This probably should then eventually replace the per-CPU copy of
mdcr_el2 that lives in debug.c, shouldn't it?

> +
>  	/* Exception Information */
>  	struct kvm_vcpu_fault_info fault;
>  
> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
> index 9d60b3006efc..657d0c94cf82 100644
> --- a/arch/arm64/include/asm/kvm_hyp.h
> +++ b/arch/arm64/include/asm/kvm_hyp.h
> @@ -95,7 +95,7 @@ void __sve_restore_state(void *sve_pffr, u32 *fpsr);
>  
>  #ifndef __KVM_NVHE_HYPERVISOR__
>  void activate_traps_vhe_load(struct kvm_vcpu *vcpu);
> -void deactivate_traps_vhe_put(void);
> +void deactivate_traps_vhe_put(struct kvm_vcpu *vcpu);
>  #endif
>  
>  u64 __guest_enter(struct kvm_vcpu *vcpu);
> diff --git a/arch/arm64/kvm/hyp/include/hyp/switch.h b/arch/arm64/kvm/hyp/include/hyp/switch.h
> index e4a2f295a394..a0e78a6027be 100644
> --- a/arch/arm64/kvm/hyp/include/hyp/switch.h
> +++ b/arch/arm64/kvm/hyp/include/hyp/switch.h
> @@ -92,11 +92,15 @@ static inline void __activate_traps_common(struct kvm_vcpu *vcpu)
>  		write_sysreg(0, pmselr_el0);
>  		write_sysreg(ARMV8_PMU_USERENR_MASK, pmuserenr_el0);
>  	}
> +
> +	vcpu->arch.mdcr_el2_host = read_sysreg(mdcr_el2);
>  	write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
>  }
>  
> -static inline void __deactivate_traps_common(void)
> +static inline void __deactivate_traps_common(struct kvm_vcpu *vcpu)
>  {
> +	write_sysreg(vcpu->arch.mdcr_el2_host, mdcr_el2);
> +
>  	write_sysreg(0, hstr_el2);
>  	if (kvm_arm_support_pmu_v3())
>  		write_sysreg(0, pmuserenr_el0);
> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> index f7af9688c1f7..2ea764a48958 100644
> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> @@ -69,12 +69,10 @@ static void __activate_traps(struct kvm_vcpu *vcpu)
>  static void __deactivate_traps(struct kvm_vcpu *vcpu)
>  {
>  	extern char __kvm_hyp_host_vector[];
> -	u64 mdcr_el2, cptr;
> +	u64 cptr;
>  
>  	___deactivate_traps(vcpu);
>  
> -	mdcr_el2 = read_sysreg(mdcr_el2);
> -
>  	if (cpus_have_final_cap(ARM64_WORKAROUND_SPECULATIVE_AT)) {
>  		u64 val;
>  
> @@ -92,13 +90,12 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
>  		isb();
>  	}
>  
> -	__deactivate_traps_common();
> +	vcpu->arch.mdcr_el2_host &= MDCR_EL2_HPMN_MASK |
> +				    MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT |
> +				    MDCR_EL2_E2TB_MASK << MDCR_EL2_E2TB_SHIFT;
>  
> -	mdcr_el2 &= MDCR_EL2_HPMN_MASK;
> -	mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;
> -	mdcr_el2 |= MDCR_EL2_E2TB_MASK << MDCR_EL2_E2TB_SHIFT;
> +	__deactivate_traps_common(vcpu);
>  
> -	write_sysreg(mdcr_el2, mdcr_el2);

FWIW, I found this whole sequence massively confusing, and it is only
when I came to patch #7 that the various pieces did come together.

Thanks,

	M.

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

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