Subject: Re: [PATCH v2 23/24] kvm: arm64: Trap host SMCs in protected mode.

From: Marc Zyngier <[email-masked]>
Date: Mon, 23 Nov 2020 17:36:16 +0000

On Mon, 16 Nov 2020 20:43:17 +0000,
David Brazdil <[email-masked]> wrote:
> 
> While protected nVHE KVM is installed, start trapping all host SMCs.
> By default, these are simply forwarded to EL3, but PSCI SMCs are
> validated first.
> 
> Create new constant HCR_HOST_NVHE_PROTECTED_FLAGS with the new set of HCR
> flags to use while the nVHE vector is installed when the kernel was
> booted with the protected flag enabled. Switch back to the default HCR
> flags when switching back to the stub vector.
> 
> Signed-off-by: David Brazdil <[email-masked]>
> ---
>  arch/arm64/include/asm/kvm_arm.h   |  1 +
>  arch/arm64/kvm/hyp/nvhe/hyp-init.S | 12 ++++++++++++
>  arch/arm64/kvm/hyp/nvhe/switch.c   |  5 ++++-
>  3 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 64ce29378467..4e90c2debf70 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -80,6 +80,7 @@
>  			 HCR_FMO | HCR_IMO | HCR_PTW )
>  #define HCR_VIRT_EXCP_MASK (HCR_VSE | HCR_VI | HCR_VF)
>  #define HCR_HOST_NVHE_FLAGS (HCR_RW | HCR_API | HCR_APK | HCR_ATA)
> +#define HCR_HOST_NVHE_PROTECTED_FLAGS (HCR_HOST_NVHE_FLAGS | HCR_TSC)
>  #define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H)
>  
>  /* TCR_EL2 Registers bits */
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-init.S b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> index 6d8202d2bdfb..8f3602f320ac 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-init.S
> @@ -88,6 +88,12 @@ SYM_CODE_END(__kvm_hyp_init)
>   * x0: struct kvm_nvhe_init_params PA
>   */
>  SYM_CODE_START(___kvm_hyp_init)
> +alternative_if ARM64_PROTECTED_KVM
> +	mov_q	x1, HCR_HOST_NVHE_PROTECTED_FLAGS
> +	msr	hcr_el2, x1
> +	isb

Why the ISB? For HCR_TSC to have any effect, you'll have to go via an
ERET to EL1 first, which will have the required synchronisation effect.

> +alternative_else_nop_endif
> +
>  	ldr	x1, [x0, #NVHE_INIT_TPIDR_EL2]
>  	msr	tpidr_el2, x1
>  
> @@ -224,6 +230,12 @@ reset:
>  	msr	sctlr_el2, x5
>  	isb
>  
> +alternative_if ARM64_PROTECTED_KVM
> +	mov_q	x5, HCR_HOST_NVHE_FLAGS
> +	msr	hcr_el2, x5
> +	isb

Same thing here, I believe.

> +alternative_else_nop_endif
> +
>  	/* Install stub vectors */
>  	adr_l	x5, __hyp_stub_vectors
>  	msr	vbar_el2, x5
> diff --git a/arch/arm64/kvm/hyp/nvhe/switch.c b/arch/arm64/kvm/hyp/nvhe/switch.c
> index 8ae8160bc93a..e1f8e0797144 100644
> --- a/arch/arm64/kvm/hyp/nvhe/switch.c
> +++ b/arch/arm64/kvm/hyp/nvhe/switch.c
> @@ -96,7 +96,10 @@ static void __deactivate_traps(struct kvm_vcpu *vcpu)
>  	mdcr_el2 |= MDCR_EL2_E2PB_MASK << MDCR_EL2_E2PB_SHIFT;
>  
>  	write_sysreg(mdcr_el2, mdcr_el2);
> -	write_sysreg(HCR_HOST_NVHE_FLAGS, hcr_el2);
> +	if (is_protected_kvm_enabled())
> +		write_sysreg(HCR_HOST_NVHE_PROTECTED_FLAGS, hcr_el2);
> +	else
> +		write_sysreg(HCR_HOST_NVHE_FLAGS, hcr_el2);
>  	write_sysreg(CPTR_EL2_DEFAULT, cptr_el2);
>  	write_sysreg(__kvm_hyp_host_vector, vbar_el2);
>  }
> -- 
> 2.29.2.299.gdc1121823c-goog
> 
> 

Thanks,

	M.

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

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