Re: [PATCH 05/17] KVM: arm64: Skip fixed-feature state flush for protected vCPUs

[Fuad Tabba]

On Wed, 2 Sept 2026 at 16:26, Vincent Donnefort <vdonnefort@xxxxxxxxxx> wrote:
>
> On Wed, Sep 02, 2026 at 04:05:12PM +0100, Vincent Donnefort wrote:
> > On Mon, Aug 31, 2026 at 05:34:09PM +0100, Fuad Tabba wrote:
> > > flush_hyp_vcpu() copies the host's mdcr_el2, TWI/TWE and debug state
> > > into the hyp vCPU on every entry, so for a protected vCPU the host's
> > > trap configuration overrides the one pkvm_vcpu_init_traps() computed
> > > at EL2. Move those copies into the non-protected branch, and sync the
> > > debug state back for non-protected vCPUs only: a protected guest's
> > > debug registers are hypervisor-owned, and it takes the host's TWI/TWE
> > > at vCPU load. MDCR_EL2.TDA traps only the guest's own accesses; it
> > > does not stop the world switch from loading host-supplied breakpoints
> > > and MDSCR_EL1.
> > >
> > > EL2's mdcr_el2 starts from 0, and HPMN == 0 is reserved without
> > > FEAT_HPMN0. Set HPMN to the CPU's event counter count at vCPU load,
> > > the value the host's copy carried; a protected guest's PMU accesses
> > > trap regardless.
> > >
> > > Signed-off-by: Fuad Tabba <fuad.tabba@xxxxxxxxx>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 31 +++++++++++++++++++-----------
> > >  arch/arm64/kvm/hyp/nvhe/pkvm.c     |  2 +-
> > >  2 files changed, 21 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > index ca7122b0bccdd..14e847b122abf 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > @@ -231,7 +231,6 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> > >     u8 esr_ec;
> > >
> > >     fpsimd_sve_flush();
> > > -   flush_debug_state(hyp_vcpu);
> > >
> > >     /*
> > >      * If we deal with a non-protected guest and the state is potentially
> > > @@ -241,7 +240,14 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> > >     if (!pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
> > >             if (vcpu_get_flag(host_vcpu, PKVM_HOST_STATE_DIRTY))
> > >                     flush_hyp_vcpu_state(hyp_vcpu);
> > > +
> > > +           hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE);
> > > +           hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
> > > +                                                    (HCR_TWI | HCR_TWE);
> > > +
> > > +           hyp_vcpu->vcpu.arch.mdcr_el2 = host_vcpu->arch.mdcr_el2;
> > >             hyp_vcpu->vcpu.arch.iflags = host_vcpu->arch.iflags;
> > > +           flush_debug_state(hyp_vcpu);
> > >     } else {
> > >             hyp_vcpu->vcpu.arch.ctxt = host_vcpu->arch.ctxt;
> > >     }
> > > @@ -249,17 +255,13 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> > >     /* __hyp_running_vcpu must be NULL in a guest context. */
> > >     hyp_vcpu->vcpu.arch.ctxt.__hyp_running_vcpu = NULL;
> > >
> > > -   hyp_vcpu->vcpu.arch.mdcr_el2    = host_vcpu->arch.mdcr_el2;
> > >     /*
> > > -    * HCR_EL2.VSE is host-owned (a pending virtual SError to inject), not a
> > > -    * trap-control bit, so it must flow to the hyp vCPU alongside TWI/TWE
> > > -    * for the vSError to be delivered. sync_hyp_vcpu() reflects it back.
> > > +    * A host-injected vSError is masked by the guest's own PSTATE.A, so it
> > > +    * applies to protected guests too.
> > >      */
> > > -   hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE | HCR_VSE);
> > > -   hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
> > > -                                            (HCR_TWI | HCR_TWE | HCR_VSE);
> > > -
> > > -   hyp_vcpu->vcpu.arch.vsesr_el2   = host_vcpu->arch.vsesr_el2;
> > > +   hyp_vcpu->vcpu.arch.hcr_el2 &= ~HCR_VSE;
> > > +   hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) & HCR_VSE;
> > > +   hyp_vcpu->vcpu.arch.vsesr_el2 = host_vcpu->arch.vsesr_el2;
> > >
> > >     flush_hyp_vgic_state(hyp_vcpu);
> > >
> > > @@ -288,7 +290,8 @@ static void sync_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu, u32 exit_reason)
> > >     struct kvm_vcpu *host_vcpu = hyp_vcpu->host_vcpu;
> > >
> > >     fpsimd_sve_sync(&hyp_vcpu->vcpu);
> > > -   sync_debug_state(hyp_vcpu);
> > > +   if (!pkvm_hyp_vcpu_is_protected(hyp_vcpu))
> > > +           sync_debug_state(hyp_vcpu);
> >
> > nit: We can't use the else arm below?
>
> or actually could we move the check into sync_debug_state() just like
> sync_hyp_timer_state() does?

Will do, and there's another case of this in the series that gets fixed with it.


/fuad

>
> >
> > >
> > >     if (pkvm_hyp_vcpu_is_protected(hyp_vcpu)) {
> > >             host_vcpu->arch.ctxt = hyp_vcpu->vcpu.arch.ctxt;
> > > @@ -329,6 +332,12 @@ static void handle___pkvm_vcpu_load(struct kvm_cpu_context *host_ctxt)
> > >             /* Propagate WFx trapping flags */
> > >             hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWE | HCR_TWI);
> > >             hyp_vcpu->vcpu.arch.hcr_el2 |= hcr_el2 & (HCR_TWE | HCR_TWI);
> > > +
> > > +           /* HPMN == 0 is reserved without FEAT_HPMN0. */
> > > +           if (system_supports_pmuv3())
> > > +                   u64p_replace_bits(&hyp_vcpu->vcpu.arch.mdcr_el2,
> > > +                                     FIELD_GET(ARMV8_PMU_PMCR_N, read_sysreg(pmcr_el0)),
> > > +                                     MDCR_EL2_HPMN);
> > >     } else {
> > >             memcpy(&hyp_vcpu->vcpu.arch.fgt, hyp_vcpu->host_vcpu->arch.fgt,
> > >                    sizeof(hyp_vcpu->vcpu.arch.fgt));
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/pkvm.c b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > > index 459bd9eb7e4bc..e85f13233da08 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/pkvm.c
> > > @@ -102,7 +102,7 @@ static void pvm_init_traps_mdcr(struct kvm_vcpu *vcpu)
> > >
> > >     if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, PMUVer, IMP)) {
> > >             val |= MDCR_EL2_TPM | MDCR_EL2_TPMCR;
> > > -           val &= ~(MDCR_EL2_HPME | MDCR_EL2_MTPME | MDCR_EL2_HPMN_MASK);
> > > +           val &= ~(MDCR_EL2_HPME | MDCR_EL2_MTPME);
> > >     }
> > >
> > >     if (!kvm_has_feat(kvm, ID_AA64DFR0_EL1, DebugVer, IMP))
> > > --
> > > 2.39.5
> > >
>
> --
> Vincent