linux-kernel - Re: [RFC PATCH v2 14/38] KVM: arm64: Synchronize EL1 system registers on virtual EL2 entry and exit

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Date:   Sun, 30 Jul 2017 22:00:00 +0200
From:   Christoffer Dall <cdall@...aro.org>
To:     Jintack Lim <jintack.lim@...aro.org>
Cc:     kvmarm@...ts.cs.columbia.edu, christoffer.dall@...aro.org,
        marc.zyngier@....com, corbet@....net, pbonzini@...hat.com,
        rkrcmar@...hat.com, linux@...linux.org.uk, catalin.marinas@....com,
        will.deacon@....com, akpm@...ux-foundation.org, mchehab@...nel.org,
        cov@...eaurora.org, daniel.lezcano@...aro.org,
        david.daney@...ium.com, mark.rutland@....com,
        suzuki.poulose@....com, stefan@...lo-penguin.com,
        andy.gross@...aro.org, wcohen@...hat.com,
        ard.biesheuvel@...aro.org, shankerd@...eaurora.org,
        vladimir.murzin@....com, james.morse@....com,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC PATCH v2 14/38] KVM: arm64: Synchronize EL1 system
 registers on virtual EL2 entry and exit

On Tue, Jul 18, 2017 at 11:58:40AM -0500, Jintack Lim wrote:
> When running in virtual EL2 we use the shadow EL1 systerm register array
> for the save/restore process, so that hardware and especially the memory
> subsystem behaves as code written for EL2 expects while really running
> in EL1.
> 
> This works great for EL1 system register accesses that we trap, because
> these accesses will be written into the virtual state for the EL1 system
> registers used when eventually switching the VCPU mode to EL1.
> 
> However, there was a collection of EL1 system registers which we do not
> trap, and as a consequence all save/restore operations of these
> registers were happening locally in the shadow array, with no benefit to
> software actually running in virtual EL1 at all.
> 
> To fix this, simply synchronize the shadow and real EL1 state for these
> registers on entry/exit to/from virtual EL2 state.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@...aro.org>
> Signed-off-by: Jintack Lim <jintack.lim@...aro.org>
> ---
>  arch/arm64/kvm/context.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 56 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/kvm/context.c b/arch/arm64/kvm/context.c
> index e965049..e1bc753 100644
> --- a/arch/arm64/kvm/context.c
> +++ b/arch/arm64/kvm/context.c
> @@ -86,6 +86,58 @@ static void flush_shadow_el1_sysregs(struct kvm_vcpu *vcpu)
>  	s_sys_regs[CPACR_EL1] = cptr_to_cpacr(vcpu_sys_reg(vcpu, CPTR_EL2));
>  }
>  
> +
> +/*
> + * List of EL0 and EL1 registers which we allow the virtual EL2 mode to access
> + * directly without trapping. This is possible because the impact of
> + * accessing those registers are the same regardless of the exception
> + * levels that are allowed.

I don't understand this last sentence...

> + */
> +static const int el1_non_trap_regs[] = {
> +	CNTKCTL_EL1,
> +	CSSELR_EL1,
> +	PAR_EL1,
> +	TPIDR_EL0,
> +	TPIDR_EL1,
> +	TPIDRRO_EL0
> +};
> +
> +/**
> + * copy_shadow_non_trap_el1_state
> + * @vcpu:      The VCPU pointer
> + * @setup:     True, if on the way to the guest (called from setup)

should setup be called flush?

then we could do

	if (flush) {
		...
	} else { /* sync */
		...
	}

> + *             False, if returning form the guet (calld from restore)

called

> + *
> + * Some EL1 registers are accessed directly by the virtual EL2 mode because
> + * they in no way affect execution state in virtual EL2.   However, we must
> + * still ensure that virtual EL2 observes the same state of the EL1 registers
> + * as the normal VM's EL1 mode, so copy this state as needed on setup/restore.
> + */

Perhaps this could be written more clearly as:

/*
 * Synchronize the state of EL1 registers directly accessible by virtual
 * EL2 between the shadow sys_regs array and the VCPU's EL1 state
 * before/after the world switch code copies the shadow state to/from
 * hardware registers.
 */

> +static void copy_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu, bool setup)
> +{
> +	u64 *s_sys_regs = vcpu->arch.ctxt.shadow_sys_regs;
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(el1_non_trap_regs); i++) {
> +		const int sr = el1_non_trap_regs[i];
> +
> +		if (setup)
> +			s_sys_regs[sr] = vcpu_sys_reg(vcpu, sr);
> +		else
> +			vcpu_sys_reg(vcpu, sr) = s_sys_regs[sr];
> +	}
> +}
> +
> +static void sync_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu)
> +{
> +	copy_shadow_non_trap_el1_state(vcpu, false);
> +}
> +
> +static void flush_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu)
> +{
> +	copy_shadow_non_trap_el1_state(vcpu, true);
> +}
> +
>  static void flush_shadow_special_regs(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
> @@ -162,6 +214,7 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>  	if (unlikely(vcpu_mode_el2(vcpu))) {
>  		flush_shadow_special_regs(vcpu);
>  		flush_shadow_el1_sysregs(vcpu);
> +		flush_shadow_non_trap_el1_state(vcpu);
>  		ctxt->hw_sys_regs = ctxt->shadow_sys_regs;
>  	} else {
>  		flush_special_regs(vcpu);
> @@ -176,9 +229,10 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
>   */
>  void kvm_arm_restore_shadow_state(struct kvm_vcpu *vcpu)
>  {
> -	if (unlikely(vcpu_mode_el2(vcpu)))
> +	if (unlikely(vcpu_mode_el2(vcpu))) {
>  		sync_shadow_special_regs(vcpu);
> -	else
> +		sync_shadow_non_trap_el1_state(vcpu);
> +	} else
>  		sync_special_regs(vcpu);
>  }
>  
> -- 
> 1.9.1
>