Re: [PATCH v2 2/2] KVM: arm64: Bound used_lrs when flushing the pKVM hyp vCPU

[Marc Zyngier <maz@kernel.org>]

On Sat, 06 Jun 2026 09:52:08 +0100,
Hyunwoo Kim <imv4bel@gmail.com> wrote:
> 
> On Fri, Jun 05, 2026 at 09:58:09AM +0100, Marc Zyngier wrote:
> > On Thu, 04 Jun 2026 16:12:03 +0100,
> > Hyunwoo Kim <imv4bel@gmail.com> wrote:
> > > 
> > > flush_hyp_vcpu() copies the host vGIC state into the hyp's private vCPU
> > > on every run. The vGIC list register save and restore use used_lrs as
> > > their loop bound and expect it to stay within the number of implemented
> > > list registers. While this is generally the case, flush_hyp_vcpu()
> > > copies vgic_v3 verbatim and does not enforce this, so a value provided
> > > by the host is used at EL2 to index vgic_lr[] and access ICH_LR<n>_EL2
> > > (host -> EL2).
> > > 
> > > Fix by clamping used_lrs to the number of implemented list registers
> > > after the copy, as the trusted path already does in
> > > vgic_flush_lr_state().
> > > 
> > > Fixes: be66e67f1750 ("KVM: arm64: Use the pKVM hyp vCPU structure in handle___kvm_vcpu_run()")
> > > Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
> > > ---
> > >  arch/arm64/kvm/hyp/nvhe/hyp-main.c | 8 ++++++++
> > >  1 file changed, 8 insertions(+)
> > > 
> > > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > index 02c5d6e5abcbf..cd807fdb11ba8 100644
> > > --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> > > @@ -7,6 +7,7 @@
> > >  #include <hyp/adjust_pc.h>
> > >  #include <hyp/switch.h>
> > >  
> > > +#include <asm/arch_gicv3.h>
> > >  #include <asm/pgtable-types.h>
> > >  #include <asm/kvm_asm.h>
> > >  #include <asm/kvm_emulate.h>
> > > @@ -142,6 +143,13 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> > >  
> > >  	hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3 = host_vcpu->arch.vgic_cpu.vgic_v3;
> > >  
> > > +	/* Bound used_lrs by the number of implemented list registers. */
> > > +	if (static_branch_unlikely(&kvm_vgic_global_state.gicv3_cpuif))
> > 
> > There is no pKVM support without a GICv3 CPU interface, and absolutely
> > everything already assumes it. Why do we need this extra check?
> > 
> > > +		hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs =
> > > +			min_t(unsigned int,
> > > +			      hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs,
> > > +			      (read_gicreg(ICH_VTR_EL2) & 0xf) + 1);
> > > +
> > 
> > Reading ICH_VTR_EL2 on each entry is going to cause some really heavy
> > trapping under NV, and we should avoid this.
> > 
> > kvm_vgic_global_state.nr_lr contains this information, and it should
> > only be a matter of replicating it (or compute it once) at init time.
> 
> Does this approach look reasonable to you?
> 
> ---
> 
> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
> index 8d06b62e7188..25199769a1d6 100644
> --- a/arch/arm64/include/asm/kvm_hyp.h
> +++ b/arch/arm64/include/asm/kvm_hyp.h
> @@ -157,5 +157,6 @@ extern unsigned long kvm_nvhe_sym(__icache_flags);
>  extern unsigned int kvm_nvhe_sym(kvm_arm_vmid_bits);
>  extern unsigned int kvm_nvhe_sym(kvm_host_sve_max_vl);
>  extern unsigned long kvm_nvhe_sym(hyp_nr_cpus);
> +extern unsigned int kvm_nvhe_sym(hyp_vgic_nr_lr);
> 
>  #endif /* __ARM64_KVM_HYP_H__ */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 9453321ef8c6..891fe2c7b854 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -2426,6 +2426,7 @@ static int __init init_subsystems(void)
>  	switch (err) {
>  	case 0:
>  		vgic_present = true;
> +		kvm_nvhe_sym(hyp_vgic_nr_lr) = kvm_vgic_global_state.nr_lr;

You probably want to guard this on kvm_vgic_global_state.gicv3_cpuif
to be sure this stays to 0 when running with a GICv2 implementation.

And maybe rename it to hyp_gicv3_nr_lr to avoid any confusion.

>  		break;
>  	case -ENODEV:
>  	case -ENXIO:
> diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> index 06db299c37a8..8bb9362bc284 100644
> --- a/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> +++ b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> @@ -128,6 +128,9 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> 
>  	hyp_vcpu->vcpu.arch.ctxt	= host_vcpu->arch.ctxt;
> 
> +	/* A guest context must keep a NULL __hyp_running_vcpu. */
> +	hyp_vcpu->vcpu.arch.ctxt.__hyp_running_vcpu = NULL;
> +
>  	hyp_vcpu->vcpu.arch.mdcr_el2	= host_vcpu->arch.mdcr_el2;
>  	hyp_vcpu->vcpu.arch.hcr_el2 &= ~(HCR_TWI | HCR_TWE);
>  	hyp_vcpu->vcpu.arch.hcr_el2 |= READ_ONCE(host_vcpu->arch.hcr_el2) &
> @@ -139,6 +142,12 @@ static void flush_hyp_vcpu(struct pkvm_hyp_vcpu *hyp_vcpu)
> 
>  	hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3 = host_vcpu->arch.vgic_cpu.vgic_v3;
> 
> +	/* Bound used_lrs by the number of implemented list registers. */
> +	hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs =
> +		min_t(unsigned int,
> +		      hyp_vcpu->vcpu.arch.vgic_cpu.vgic_v3.used_lrs,
> +		      hyp_vgic_nr_lr);
> +
>  	hyp_vcpu->vcpu.arch.pid = host_vcpu->arch.pid;
>  }
> 
> diff --git a/arch/arm64/kvm/hyp/nvhe/setup.c b/arch/arm64/kvm/hyp/nvhe/setup.c
> index d461981616d9..ebc6b4afc336 100644
> --- a/arch/arm64/kvm/hyp/nvhe/setup.c
> +++ b/arch/arm64/kvm/hyp/nvhe/setup.c
> @@ -20,6 +20,7 @@
>  #include <nvhe/trap_handler.h>
> 
>  unsigned long hyp_nr_cpus;
> +unsigned int hyp_vgic_nr_lr;

You could probably move this close to the place where it is used
instead of an unrelated file.

Other than that, and with the vcpu-related hunk in its own patch, this
looks reasonable.

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.