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

Alexandru Elisei
2022-02-04T15:41:34Z

Hi Marc,

On Fri, Jan 28, 2022 at 12:18:30PM +0000, Marc Zyngier wrote:
> From: Jintack Lim <jintack.lim@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@linaro.org>
> Signed-off-by: Marc Zyngier <maz@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@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
>