Subject: Re: [PATCH 2/3] KVM: arm64: Stop save/restoring ACTLR_EL1

From: Marc Zyngier <[email-masked]>
Date: Thu, 28 May 2020 13:36:14 +0100

On 2020-05-26 17:18, James Morse wrote:
> KVM sets HCR_EL2.TACR (which it calls HCR_TAC) via HCR_GUEST_FLAGS.

TAC is a leftover from 32bit.

> This means ACTLR* accesses from the guest are always trapped, and
> always return the value in the sys_regs array.
> 
> The guest can't change the value of these registers, so we are
> save restoring the reset value, which came from the host.
> 
> Stop save/restoring this register.
> 
> This also stops this register being affected by sysregs_loaded_on_cpu,
> so we can provide 32 bit accessors that always use the in-memory copy.
> 
> Signed-off-by: James Morse <[email-masked]>
> ---
>  arch/arm64/kvm/hyp/sysreg-sr.c | 2 --
>  arch/arm64/kvm/sys_regs.c      | 2 --
>  2 files changed, 4 deletions(-)
> 
> diff --git a/arch/arm64/kvm/hyp/sysreg-sr.c 
> b/arch/arm64/kvm/hyp/sysreg-sr.c
> index 75b1925763f1..57116cf3a1a5 100644
> --- a/arch/arm64/kvm/hyp/sysreg-sr.c
> +++ b/arch/arm64/kvm/hyp/sysreg-sr.c
> @@ -44,7 +44,6 @@ static void __hyp_text
> __sysreg_save_el1_state(struct kvm_cpu_context *ctxt)
>  {
>  	ctxt->sys_regs[CSSELR_EL1]	= read_sysreg(csselr_el1);
>  	ctxt->sys_regs[SCTLR_EL1]	= read_sysreg_el1(SYS_SCTLR);
> -	ctxt->sys_regs[ACTLR_EL1]	= read_sysreg(actlr_el1);
>  	ctxt->sys_regs[CPACR_EL1]	= read_sysreg_el1(SYS_CPACR);
>  	ctxt->sys_regs[TTBR0_EL1]	= read_sysreg_el1(SYS_TTBR0);
>  	ctxt->sys_regs[TTBR1_EL1]	= read_sysreg_el1(SYS_TTBR1);
> @@ -133,7 +132,6 @@ static void __hyp_text
> __sysreg_restore_el1_state(struct kvm_cpu_context *ctxt)
>  		isb();
>  	}
> 
> -	write_sysreg(ctxt->sys_regs[ACTLR_EL1],		actlr_el1);

If we don't need to save/restore it, we can also drop its presence
in the sys_regs array.

>  	write_sysreg_el1(ctxt->sys_regs[CPACR_EL1],	SYS_CPACR);
>  	write_sysreg_el1(ctxt->sys_regs[TTBR0_EL1],	SYS_TTBR0);
>  	write_sysreg_el1(ctxt->sys_regs[TTBR1_EL1],	SYS_TTBR1);
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 2eda539f3281..aae58513025c 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -81,7 +81,6 @@ u64 vcpu_read_sys_reg(const struct kvm_vcpu *vcpu, 
> int reg)
>  	switch (reg) {
>  	case CSSELR_EL1:	return read_sysreg_s(SYS_CSSELR_EL1);
>  	case SCTLR_EL1:		return read_sysreg_s(SYS_SCTLR_EL12);
> -	case ACTLR_EL1:		return read_sysreg_s(SYS_ACTLR_EL1);
>  	case CPACR_EL1:		return read_sysreg_s(SYS_CPACR_EL12);
>  	case TTBR0_EL1:		return read_sysreg_s(SYS_TTBR0_EL12);
>  	case TTBR1_EL1:		return read_sysreg_s(SYS_TTBR1_EL12);
> @@ -124,7 +123,6 @@ void vcpu_write_sys_reg(struct kvm_vcpu *vcpu, u64
> val, int reg)
>  	switch (reg) {
>  	case CSSELR_EL1:	write_sysreg_s(val, SYS_CSSELR_EL1);	return;
>  	case SCTLR_EL1:		write_sysreg_s(val, SYS_SCTLR_EL12);	return;
> -	case ACTLR_EL1:		write_sysreg_s(val, SYS_ACTLR_EL1);	return;
>  	case CPACR_EL1:		write_sysreg_s(val, SYS_CPACR_EL12);	return;
>  	case TTBR0_EL1:		write_sysreg_s(val, SYS_TTBR0_EL12);	return;
>  	case TTBR1_EL1:		write_sysreg_s(val, SYS_TTBR1_EL12);	return;

It strikes me that we don't even have a trap handler for this sysreg,
whether it is 32 or 64bit... That's a bit unfortunate, to say the
least...

         M.
-- 
Jazz is not dead. It just smells funny...

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