[PATCH] KVM: arm64: Force HCR_EL2.xMO to 1 at all times in VHE mode

[PATCH] KVM: arm64: Force HCR_EL2.xMO to 1 at all times in VHE mode

[Marc Zyngier]

We keep setting and clearing these bits depending on the role of
the host kernel, mimicking what we do for nVHE. But that's actually
pretty pointless, as we always want physical interrupts to make it
to the host, at EL2.

This has also two problems:

- it prevents IRQs from being taken when these bits are cleared
  if the implementation has chosen to implement these bits as
  masks when HCR_EL2.{TGE,xMO}=={0,0}

- it triggers a bad erratum on the AmpereOne HW, which catches
  fire on clearing these bits while an interrupt is being taken
  (AC03_CPU_36).

Let's kill these two birds with a single stone.

Reported-by: D Scott Phillips <scott@os.amperecomputing.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
---
 arch/arm64/include/asm/kvm_arm.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
index 974d72b5905b8..bba4b0e930915 100644
--- a/arch/arm64/include/asm/kvm_arm.h
+++ b/arch/arm64/include/asm/kvm_arm.h
@@ -100,7 +100,7 @@
 			 HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 | HCR_TID1)
 #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)
+#define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H | HCR_AMO | HCR_IMO | HCR_FMO)
 
 #define HCRX_HOST_FLAGS (HCRX_EL2_MSCEn | HCRX_EL2_TCR2En | HCRX_EL2_EnFPM)
 #define MPAMHCR_HOST_FLAGS	0
-- 
2.39.2

Re: [PATCH] KVM: arm64: Force HCR_EL2.xMO to 1 at all times in VHE mode

[D Scott Phillips]

Marc Zyngier <maz@kernel.org> writes:

> We keep setting and clearing these bits depending on the role of
> the host kernel, mimicking what we do for nVHE. But that's actually
> pretty pointless, as we always want physical interrupts to make it
> to the host, at EL2.
>
> This has also two problems:
>
> - it prevents IRQs from being taken when these bits are cleared
>   if the implementation has chosen to implement these bits as
>   masks when HCR_EL2.{TGE,xMO}=={0,0}
>
> - it triggers a bad erratum on the AmpereOne HW, which catches
>   fire on clearing these bits while an interrupt is being taken
>   (AC03_CPU_36).
>
> Let's kill these two birds with a single stone.
>
> Reported-by: D Scott Phillips <scott@os.amperecomputing.com>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_arm.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> index 974d72b5905b8..bba4b0e930915 100644
> --- a/arch/arm64/include/asm/kvm_arm.h
> +++ b/arch/arm64/include/asm/kvm_arm.h
> @@ -100,7 +100,7 @@
>  			 HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 | HCR_TID1)
>  #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)
> +#define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H | HCR_AMO | HCR_IMO | HCR_FMO)
>  
>  #define HCRX_HOST_FLAGS (HCRX_EL2_MSCEn | HCRX_EL2_TCR2En | HCRX_EL2_EnFPM)
>  #define MPAMHCR_HOST_FLAGS	0

Should the xMO twiddling in __vgic_v3_get_gic_config() also get dropped
here?

Re: [PATCH] KVM: arm64: Force HCR_EL2.xMO to 1 at all times in VHE mode

[Marc Zyngier]

On Thu, 24 Apr 2025 23:24:59 +0100,
D Scott Phillips <scott@os.amperecomputing.com> wrote:
> 
> Marc Zyngier <maz@kernel.org> writes:
> 
> > We keep setting and clearing these bits depending on the role of
> > the host kernel, mimicking what we do for nVHE. But that's actually
> > pretty pointless, as we always want physical interrupts to make it
> > to the host, at EL2.
> >
> > This has also two problems:
> >
> > - it prevents IRQs from being taken when these bits are cleared
> >   if the implementation has chosen to implement these bits as
> >   masks when HCR_EL2.{TGE,xMO}=={0,0}
> >
> > - it triggers a bad erratum on the AmpereOne HW, which catches
> >   fire on clearing these bits while an interrupt is being taken
> >   (AC03_CPU_36).
> >
> > Let's kill these two birds with a single stone.
> >
> > Reported-by: D Scott Phillips <scott@os.amperecomputing.com>
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/include/asm/kvm_arm.h | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_arm.h b/arch/arm64/include/asm/kvm_arm.h
> > index 974d72b5905b8..bba4b0e930915 100644
> > --- a/arch/arm64/include/asm/kvm_arm.h
> > +++ b/arch/arm64/include/asm/kvm_arm.h
> > @@ -100,7 +100,7 @@
> >  			 HCR_FMO | HCR_IMO | HCR_PTW | HCR_TID3 | HCR_TID1)
> >  #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)
> > +#define HCR_HOST_VHE_FLAGS (HCR_RW | HCR_TGE | HCR_E2H | HCR_AMO | HCR_IMO | HCR_FMO)
> >  
> >  #define HCRX_HOST_FLAGS (HCRX_EL2_MSCEn | HCRX_EL2_TCR2En | HCRX_EL2_EnFPM)
> >  #define MPAMHCR_HOST_FLAGS	0
> 
> Should the xMO twiddling in __vgic_v3_get_gic_config() also get dropped
> here?

Ah, that old chestnut. Yeah, well spotted. It could do with some
TLC. I'll do some surgery on it and post a v2.

Thanks,

	M.

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