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

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Marc,

On Fri, Feb 04, 2022 at 03:40:15PM +0000, Alexandru Elisei wrote:
> 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?

Or kvm_handle_wtf() can be rewritten to use forward_traps() introduced in the patch after the
next one (#24, "KVM: arm64: nv: Respect the virtual HCR_EL2.NV bit setting"):

static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
{
	bool is_wfe = !!(kvm_vcpu_get_esr(vcpu) & ESR_ELx_WFx_ISS_WFE);

	if (is_wfe && forward_traps(vcpu, HCR_TWE))
		return 1;

	if (!is_wfe && forward_traps(vcpu, HCR_TWI))
		return 1;

	[..]
}

Plenty of options to choose from.

Thanks,
Alex

> 
> 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
> >