[PATCH v6 22/64] KVM: arm64: nv: Respect virtual HCR_EL2.TWX setting

[Alexandru Elisei]

Hi Marc,

On Fri, Jan 28, 2022 at 12:18:30PM +0000, Marc Zyngier wrote:
> From: Jintack Lim <jintack.lim at linaro.org>
> 
> Forward exceptions due to WFI or WFE instructions to the virtual EL2 if
> they are not coming from the virtual EL2 and virtual HCR_EL2.TWX is set.
> 
> Signed-off-by: Jintack Lim <jintack.lim at linaro.org>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/include/asm/kvm_nested.h |  2 ++
>  arch/arm64/kvm/Makefile             |  2 +-
>  arch/arm64/kvm/handle_exit.c        | 11 ++++++++++-
>  arch/arm64/kvm/nested.c             | 28 ++++++++++++++++++++++++++++
>  4 files changed, 41 insertions(+), 2 deletions(-)
>  create mode 100644 arch/arm64/kvm/nested.c
> 
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index 5a85be6d8eb3..79d382fa02ea 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -65,4 +65,6 @@ static inline u64 translate_cnthctl_el2_to_cntkctl_el1(u64 cnthctl)
>  		(cnthctl & (CNTHCTL_EVNTI | CNTHCTL_EVNTDIR | CNTHCTL_EVNTEN)));
>  }
>  
> +int handle_wfx_nested(struct kvm_vcpu *vcpu, bool is_wfe);
> +
>  #endif /* __ARM64_KVM_NESTED_H */
> diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
> index b67c4ebd72b1..dbaf42ff65f1 100644
> --- a/arch/arm64/kvm/Makefile
> +++ b/arch/arm64/kvm/Makefile
> @@ -14,7 +14,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
>  	 inject_fault.o va_layout.o handle_exit.o \
>  	 guest.o debug.o reset.o sys_regs.o \
>  	 vgic-sys-reg-v3.o fpsimd.o pmu.o pkvm.o \
> -	 arch_timer.o trng.o emulate-nested.o \
> +	 arch_timer.o trng.o emulate-nested.o nested.o \
>  	 vgic/vgic.o vgic/vgic-init.o \
>  	 vgic/vgic-irqfd.o vgic/vgic-v2.o \
>  	 vgic/vgic-v3.o vgic/vgic-v4.o \
> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
> index 0cedef6e0d80..a1b1bbf3d598 100644
> --- a/arch/arm64/kvm/handle_exit.c
> +++ b/arch/arm64/kvm/handle_exit.c
> @@ -119,7 +119,16 @@ static int handle_no_fpsimd(struct kvm_vcpu *vcpu)
>   */
>  static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
>  {
> -	if (kvm_vcpu_get_esr(vcpu) & ESR_ELx_WFx_ISS_WFE) {
> +	bool is_wfe = !!(kvm_vcpu_get_esr(vcpu) & ESR_ELx_WFx_ISS_WFE);
> +
> +	if (vcpu_has_nv(vcpu)) {
> +		int ret = handle_wfx_nested(vcpu, is_wfe);
> +
> +		if (ret != -EINVAL)
> +			return ret;

I find this rather clunky. The common pattern is that a function returns
early when it encounters an error, but here this pattern is reversed:
-EINVAL means that handle_wfx_nested() failed in handling the WFx, so
proceed as usual; conversly, anything but -EINVAL means handle_wfx_nested()
was successful in handling WFx, so exit early from kvm_handle_wfx().

That would be ok by itself, but if we dig deeper, handle_wfx_nested() ends up
calling kvm_inject_nested(), where -EINVAL is actually an error code. Granted,
that should never happen, because kvm_handle_wfx() first checks vcpu_has_nv(),
but still feels like something that could be improved.

Maybe changing handle_wfx_nested() like this would be better:

--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -14,15 +14,18 @@
  * the virtual HCR_EL2.TWX is set. Otherwise, let the host hypervisor
  * handle this.
  */
-int handle_wfx_nested(struct kvm_vcpu *vcpu, bool is_wfe)
+bool handle_wfx_nested(struct kvm_vcpu *vcpu, bool is_wfe, int *error)
 {
        u64 hcr_el2 = __vcpu_sys_reg(vcpu, HCR_EL2);
 
+       *error = 0;
        if (vcpu_is_el2(vcpu))
-               return -EINVAL;
+               return false;
 
-       if ((is_wfe && (hcr_el2 & HCR_TWE)) || (!is_wfe && (hcr_el2 & HCR_TWI)))
-               return kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+       if ((is_wfe && (hcr_el2 & HCR_TWE)) || (!is_wfe && (hcr_el2 & HCR_TWI))) {
+               *error = kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
+               return true;
+       }
 
-       return -EINVAL;
+       return false;
 }

Now the return value means one thing only (did handle_wfx_nested() handle
the trap?) and we still capture the error code.

Or perhaps folding handle_wfx_nested() into kvm_handle_wfx() would be
preferable.

What do you think?

Thanks,
Alex

> +	}
> +
> +	if (is_wfe) {
>  		trace_kvm_wfx_arm64(*vcpu_pc(vcpu), true);
>  		vcpu->stat.wfe_exit_stat++;
>  		kvm_vcpu_on_spin(vcpu, vcpu_mode_priv(vcpu));
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> new file mode 100644
> index 000000000000..5e1104f8e765
> --- /dev/null
> +++ b/arch/arm64/kvm/nested.c
> @@ -0,0 +1,28 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2017 - Columbia University and Linaro Ltd.
> + * Author: Jintack Lim <jintack.lim at linaro.org>
> + */
> +
> +#include <linux/kvm.h>
> +#include <linux/kvm_host.h>
> +
> +#include <asm/kvm_emulate.h>
> +
> +/*
> + * Inject wfx to the virtual EL2 if this is not from the virtual EL2 and
> + * the virtual HCR_EL2.TWX is set. Otherwise, let the host hypervisor
> + * handle this.
> + */
> +int handle_wfx_nested(struct kvm_vcpu *vcpu, bool is_wfe)
> +{
> +	u64 hcr_el2 = __vcpu_sys_reg(vcpu, HCR_EL2);
> +
> +	if (vcpu_is_el2(vcpu))
> +		return -EINVAL;
> +
> +	if ((is_wfe && (hcr_el2 & HCR_TWE)) || (!is_wfe && (hcr_el2 & HCR_TWI)))
> +		return kvm_inject_nested_sync(vcpu, kvm_vcpu_get_esr(vcpu));
> +
> +	return -EINVAL;
> +}
> -- 
> 2.30.2
>