Re: [RFC PATCH v2 11/15] KVM: arm64: Export stage-2 MMU config

[Marc Zyngier <maz@kernel.org>]

On Thu, 15 Oct 2026 15:25:45 +0100,
Ryan Roberts <ryan.roberts@arm.com> wrote:
> 
> From: Jean-Philippe Brucker <jpb@kernel.org>
> 
> Provide an API to drivers that want to share the stage 2 configuration
> of a VM with their device. At the moment it is rather limited:
> 
> * No invalidation notifiers. The TLBs on the device side, if any,
>   receive CPU TLB invalidations.
> 
> * No remote page fault support. All paging still takes the normal data
>   abort route on the CPU. Options are pinning all guest memory or
>   device-specific mechanism for reporting pending faults to the guest.
> 
> * No nested VMs. Although we could perhaps support assigning devices to
>   the hyp context of a nested VM, it seems performative.
> 
> So for the moment this API works for devices that share the CPU MMU, but
> the SMMU driver could use it with DVM.
> 
> Signed-off-by: Jean-Philippe Brucker <jpb@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_host.h |  11 ++++
>  arch/arm64/include/asm/kvm_mmu.h  |   1 +
>  arch/arm64/kvm/mmu.c              | 102 ++++++++++++++++++++++++++++++
>  arch/arm64/kvm/sys_regs.c         |  14 +---
>  4 files changed, 117 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 30d0082b15ec..c0932b43f959 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -235,6 +235,14 @@ struct kvm_s2_mmu {
>  	atomic_t refcnt;
>  };
>  
> +/* stage 2 MMU configuration, for drivers that speak VMSA */
> +struct kvm_arch_guest_mmu {
> +	u64 hcr;
> +	u64 hcrx;
> +	u64 vtcr;
> +	u64 vttbr;
> +};
> +
>  struct kvm_arch_memory_slot {
>  };
>  
> @@ -1720,4 +1728,7 @@ static __always_inline enum fgt_group_id __fgt_reg_to_group_id(enum vcpu_sysreg
>  
>  long kvm_get_cap_for_kvm_ioctl(unsigned int ioctl, long *ext);
>  
> +struct kvm_arch_guest_mmu *kvm_arch_get_guest_mmu(struct kvm *kvm);
> +void kvm_arch_put_guest_mmu(struct kvm *kvm, struct kvm_arch_guest_mmu *mmu);
> +
>  #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 6eae7e7e2a68..8a9d385fe14d 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -185,6 +185,7 @@ int kvm_handle_guest_sea(struct kvm_vcpu *vcpu);
>  int kvm_handle_guest_abort(struct kvm_vcpu *vcpu);
>  
>  phys_addr_t kvm_mmu_get_httbr(void);
> +u64 kvm_mmu_get_hcr(struct kvm *kvm, u64 hcr);
>  phys_addr_t kvm_get_idmap_vector(void);
>  int __init kvm_mmu_init(u32 hyp_va_bits);
>  
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 2d44cd6a5aed..0bf1dff2a41f 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -2805,3 +2805,105 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled)
>  
>  	trace_kvm_toggle_cache(*vcpu_pc(vcpu), was_enabled, now_enabled);
>  }
> +
> +#define HCR_MMU_BITS (HCR_VM | HCR_PTW | HCR_DC | HCR_TGE | HCR_RW |	\
> +		      HCR_CD | HCR_ID | HCR_E2H | HCR_NV | HCR_NV1 |	\
> +		      HCR_FWB | HCR_ATA | HCR_DCT)
> +
> +/* Update MMU-specific host bits of HCR_EL2 */
> +u64 kvm_mmu_get_hcr(struct kvm *kvm, u64 hcr)
> +{
> +	lockdep_assert_held(&kvm->arch.config_lock);
> +
> +	WARN_ON(!test_bit(KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED,
> +			  &kvm->arch.flags));
> +
> +	hcr &= ~HCR_MMU_BITS;
> +	hcr |= HCR_VM | HCR_PTW;
> +
> +	if (has_vhe() || has_hvhe())
> +		hcr |= HCR_E2H;
> +
> +	if (cpus_have_final_cap(ARM64_HAS_STAGE2_FWB))
> +		hcr |= HCR_FWB;
> +
> +	if (kvm_has_mte(kvm))
> +		hcr |= HCR_ATA;
> +
> +	if (!test_bit(KVM_ARM_VCPU_EL1_32BIT, kvm->arch.vcpu_features))
> +		hcr |= HCR_RW;
> +
> +	return hcr;
> +}
> +
> +/**
> + * kvm_arch_get_guest_mmu - Get a reference to the guest MMU config
> + * @kvm: KVM instance. Caller must hold a reference.
> + *
> + * Get the stage 2 MMU config of an EL1 VM. The returned object contains
> + * settings that do not change until the object is released with
> + * kvm_arch_put_guest_mmu(). This function does not support nested VMs.
> + *
> + * HCR has lots of settings, some of which differ between vCPUs, but most of
> + * them control traps. For EL1 VMs, the bits relevant to MMU translation are
> + * common to all vCPUs:
> + * - VM, PTW, DC, TGE, CD, ID, NV, NV1, DCT have fixed values.
> + * - E2H, FWB, ATA depend on host CPU features (respectively VHE, FWB and MTE2).
> + * - RW depends on vCPU features. This function does not support 32-bit VMs.
> + *
> + * HCRX: EnSDERR, EnSNERR set to 0. TODO: is GCSEn relevant?
> + *
> + * VTCR is set once for the VM, depends on the VM's IPA size.
> + *
> + * VTTBR:
> + * - VMID is pinned, until kvm_arch_put_guest_mmu() is called.
> + * - PGD is allocated and freed with the vm (the caller must hold their
> + *   reference to @kvm). KVM may create and remove page table mappings at any
> + *   moment, and uses normal TLB maintenance operations to synchronize with
> + *   page table walkers. KVM does not notify the caller about mapping changes.
> + * - CNP is a property of the host system.
> + *
> + * Return: a new kvm_arch_guest_mmu instance, or an error pointer on failure.
> + */
> +struct kvm_arch_guest_mmu *kvm_arch_get_guest_mmu(struct kvm *kvm)
> +{
> +	int ret;
> +	struct kvm_arch_guest_mmu *mmu;
> +
> +	guard(mutex)(&kvm->arch.config_lock);
> +
> +	if (is_protected_kvm_enabled())
> +		return ERR_PTR(-EOPNOTSUPP);

How does this compose with CCA? Can it be restricted to exclusively
running VHE rather than also dealing with the lesser configurations?

> +
> +	if (!test_bit(KVM_ARCH_FLAG_VCPU_FEATURES_CONFIGURED, &kvm->arch.flags) ||
> +	    test_bit(KVM_ARM_VCPU_EL1_32BIT, kvm->arch.vcpu_features) ||
> +	    test_bit(KVM_ARM_VCPU_HAS_EL2, kvm->arch.vcpu_features))
> +		return ERR_PTR(-EINVAL);
> +
> +	ret = kvm_arm_pinned_vmid_get(kvm);
> +	if (ret < 0)
> +		return ERR_PTR(ret);
> +
> +	ret = -ENOMEM;
> +	mmu = kzalloc_obj(*mmu, GFP_KERNEL_ACCOUNT);
> +	if (!mmu)
> +		goto err_unpin_vmid;
> +
> +	mmu->vtcr = kvm->arch.mmu.vtcr;
> +	mmu->vttbr = kvm_get_vttbr(&kvm->arch.mmu);
> +	mmu->hcr = kvm_mmu_get_hcr(kvm, 0);

How about the hcrx field in the structure?

> +
> +	return mmu;
> +
> +err_unpin_vmid:
> +	kvm_arm_pinned_vmid_put(kvm);
> +	return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(kvm_arch_get_guest_mmu);

It really feels like this is a bit upside down. Since you are actively
pinning the S2 MMU context, it should only be a matter of taking a
reference on the kvm struct (just like KVM devices do), and handing
off the kvm_s2_mmu structure, rather than copying things. When you're
done with it, drop the reference.

If there is a need to deal with a different value for HCR_EL2 (or
anything else), it can be done in the code that makes use of it,
rather than being in the KVM code.

Thanks,

	M.

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