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

[Marc Zyngier <maz@kernel.org>]

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.