[PATCH v1 5/8] KVM: arm64: Fix Trace Buffer trap polarity for protected VMs

[Fuad Tabba]

On Tue, 4 Nov 2025 at 18:03, Suzuki K Poulose <suzuki.poulose at arm.com> wrote:
>
> On 04/11/2025 17:56, Fuad Tabba wrote:
> > Hi Suzuki,
> >
> > On Tue, 4 Nov 2025 at 17:50, Suzuki K Poulose <suzuki.poulose at arm.com> wrote:
> >>
> >> On 04/11/2025 12:59, Fuad Tabba wrote:
> >>> The E2TB bits in MDCR_EL2 control trapping of Trace Buffer system
> >>> register accesses. These accesses are trapped to EL2 when the bits are
> >>> clear.
> >>>
> >>> The trap initialization logic for protected VMs in pvm_init_traps_mdcr()
> >>> had the polarity inverted. When a guest did not support the ExtTrcBuff
> >>> feature, the code was setting E2TB. This incorrectly disabled the trap,
> >>> potentially allowing a protected guest to access registers for a feature
> >>> it was not given.
> >>>
> >>> Fix this by inverting the operation.
> >>>
> >>> Fixes: f50758260bff ("KVM: arm64: Group setting traps for protected VMs by control register")
> >>> Signed-off-by: Fuad Tabba <tabba at google.com>
> >>> ---
> >>>    arch/arm64/kvm/hyp/nvhe/pkvm.c | 2 +-
> >>>    1 file changed, 1 insertion(+), 1 deletion(-)
> >>>
> >>> diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> >>> index 43bde061b65d..df0c59a29b35 100644
> >>> --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> >>> +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> >>> @@ -118,7 +118,7 @@ static void pvm_init_traps_mdcr(struct kvm_vcpu *vcpu)
> >>>                val |= MDCR_EL2_TTRF;
> >>>
> >>>        if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, ExtTrcBuff, IMP))
> >>
> >> Why are we checkin ExtTrcBuff ? Shouldn't that be TraceBuffer ?
> >
> > This is an existing bug that needs to be fixed. Thank for spotting it.
> >
> >>> -             val |= MDCR_EL2_E2TB_MASK;
> >>> +             val &= ~MDCR_EL2_E2TB_MASK;
> >> This looks correct to me.
> >
> > I don't thin it's correct. E2TB traps when clear, not set:
> > https://developer.arm.com/documentation/ddi0601/2025-09/AArch64-Registers/MDCR-EL2--Monitor-Debug-Configuration-Register--EL2-
>
> I meant your change to val looks correct.

I see, thanks :) I'll fix the existing bug when I respin this.

Cheers,
/fuad

> Suzuki
>
> >
> >> Suzuki
> >>
> >>>
> >>>        /* Trap Debug Communications Channel registers */
> >>>        if (!kvm_has_feat(kvm, ID_AA64MMFR0_EL1, FGT, IMP))
> >>
> >>
>