linux-kernel - Re: [PATCH v4 1/6] arm64/kvm: preserve host HCR

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Date:   Fri, 4 Jan 2019 17:50:38 +0000
From:   James Morse <james.morse@....com>
To:     Amit Daniel Kachhap <amit.kachhap@....com>
Cc:     linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <marc.zyngier@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/6] arm64/kvm: preserve host HCR_EL2 value

Hi Amit,

On 18/12/2018 07:56, Amit Daniel Kachhap wrote:
> When restoring HCR_EL2 for the host, KVM uses HCR_HOST_VHE_FLAGS, which
> is a constant value. This works today, as the host HCR_EL2 value is
> always the same, but this will get in the way of supporting extensions
> that require HCR_EL2 bits to be set conditionally for the host.
> 
> To allow such features to work without KVM having to explicitly handle
> every possible host feature combination, this patch has KVM save/restore
> the host HCR when switching to/from a guest HCR. The saving of the
> register is done once during cpu hypervisor initialization state and is
> just restored after switch from guest.
> 
> For fetching HCR_EL2 during kvm initilisation, a hyp call is made using

(initialisation)


> kvm_call_hyp and is helpful in NHVE case.
> 
> For the hyp TLB maintenance code, __tlb_switch_to_host_vhe() is updated
> to toggle the TGE bit with a RMW sequence, as we already do in
> __tlb_switch_to_guest_vhe().


> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index aea01a0..25ac9fa 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -73,6 +73,8 @@ extern void __vgic_v3_init_lrs(void);
>  
>  extern u32 __kvm_get_mdcr_el2(void);
>  
> +extern u64 __read_hyp_hcr_el2(void);

How come this isn't __kvm_get_hcr_el2() like mdcr?


> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 52fbc82..1b9eed9 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -196,13 +196,17 @@ enum vcpu_sysreg {
>  
>  #define NR_COPRO_REGS	(NR_SYS_REGS * 2)
>  
> +struct kvm_cpu_init_host_regs {
> +	u64 hcr_el2;
> +};
> +
>  struct kvm_cpu_context {
>  	struct kvm_regs	gp_regs;
>  	union {
>  		u64 sys_regs[NR_SYS_REGS];
>  		u32 copro[NR_COPRO_REGS];
>  	};
> -
> +	struct kvm_cpu_init_host_regs init_regs;
>  	struct kvm_vcpu *__hyp_running_vcpu;
>  };

Hmm, so we grow every vcpu's struct kvm_cpu_context with some host-only registers...


> @@ -211,7 +215,7 @@ typedef struct kvm_cpu_context kvm_cpu_context_t;
>  struct kvm_vcpu_arch {
>  	struct kvm_cpu_context ctxt;
>  
> -	/* HYP configuration */
> +	/* Guest HYP configuration */
>  	u64 hcr_el2;
>  	u32 mdcr_el2;

... but they aren't actually host-only.


I think it would be tidier to move these two into struct kvm_cpu_context (not as
some init_host state), as both host and vcpu's have these values.
You could then add the mdcr_el2 stashing to your __cpu_copy_host_registers()
too. This way they both work in the same way, otherwise one is per-cpu, the
other is in a special bit of only the host's kvm_cpu_context.


> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
> index f6e02cc..85a2a5c 100644
> --- a/arch/arm64/kvm/hyp/switch.c
> +++ b/arch/arm64/kvm/hyp/switch.c
> @@ -139,15 +139,15 @@ static void __hyp_text __activate_traps(struct kvm_vcpu *vcpu)
>  		__activate_traps_nvhe(vcpu);
>  }
>  
> -static void deactivate_traps_vhe(void)
> +static void deactivate_traps_vhe(struct kvm_cpu_context *host_ctxt)
>  {
>  	extern char vectors[];	/* kernel exception vectors */
> -	write_sysreg(HCR_HOST_VHE_FLAGS, hcr_el2);
> +	write_sysreg(host_ctxt->init_regs.hcr_el2, hcr_el2);
>  	write_sysreg(CPACR_EL1_DEFAULT, cpacr_el1);
>  	write_sysreg(vectors, vbar_el1);
>  }
>  
> -static void __hyp_text __deactivate_traps_nvhe(void)
> +static void __hyp_text __deactivate_traps_nvhe(struct kvm_cpu_context *host_ctxt)
>  {
>  	u64 mdcr_el2 = read_sysreg(mdcr_el2);
>  
> @@ -157,12 +157,15 @@ static void __hyp_text __deactivate_traps_nvhe(void)
>  	mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;
>  
>  	write_sysreg(mdcr_el2, mdcr_el2);

Strangely we try to rebuild the host's mdcr value here. If we had the host mdcr
value in host_ctxt we could restore it directly.


> -	write_sysreg(HCR_HOST_NVHE_FLAGS, hcr_el2);
> +	write_sysreg(host_ctxt->init_regs.hcr_el2, hcr_el2);
>  	write_sysreg(CPTR_EL2_DEFAULT, cptr_el2);
>  }

>  static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
>  {
> +	struct kvm_cpu_context *host_ctxt;
> +
> +	host_ctxt = vcpu->arch.host_cpu_context;
>  	/*
>  	 * If we pended a virtual abort, preserve it until it gets
>  	 * cleared. See D1.14.3 (Virtual Interrupts) for details, but
> @@ -173,9 +176,9 @@ static void __hyp_text __deactivate_traps(struct kvm_vcpu *vcpu)
>  		vcpu->arch.hcr_el2 = read_sysreg(hcr_el2);
>  
>  	if (has_vhe())
> -		deactivate_traps_vhe();
> +		deactivate_traps_vhe(host_ctxt);
>  	else
> -		__deactivate_traps_nvhe();
> +		__deactivate_traps_nvhe(host_ctxt);
>  }

(Alternatively each of these deactivate_traps() calls could retrieve the
host_ctxt directly as its a per-cpu variable, but as we have the struct vcpu
here, this is probably better.)


Thanks,

James