Re: [RFC PATCH 5/5] KVM: arm64: Enable HAFDBS for guests not on migration

[Oliver Upton]

Hi,

On Tue, Sep 01, 2026 at 06:15:56PM +0100, Leonardo Bras wrote:
> When dirty-logging is disabled, even non-write faults make a page dirty,
> which avoids a second fault when the page is actually written to.
> 
> On dirty-logging enable, this approach causes all (writable) pages on the
> memslot to be marked clean, even if they were not written to, which can
> take a lot of time, while holding the MMU lock, doing atomic writes to
> PTEs.

Do you have any performance numbers for this? Enabling HAFDBS seems a
bit involved to avoid some stores on the first pass.

> So, if the system supports VHE + HAFDBS, keep the non-write-faulted page as
> writable-clean, and let HAFDBS update that on demand when a write happens.
> 
> When dirty-tracking actually starts, disable HAFDBS as having it on
> avoids the same fault that is used for dirty-logging.
> 
> Signed-off-by: Leonardo Bras <leo.bras@xxxxxxx>
> ---
>  arch/arm64/include/asm/kvm_mmu.h    |  6 ++++++
>  arch/arm64/include/asm/kvm_nested.h |  9 +++++++--
>  arch/arm64/kvm/arm.c                |  7 +++++++
>  arch/arm64/kvm/mmu.c                | 26 +++++++++++++++++++++++++-
>  4 files changed, 45 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 6eae7e7e2a68..3defa1a988d3 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -383,20 +383,26 @@ static inline void kvm_fault_unlock(struct kvm *kvm)
>   * and CMOs are NOP'd. This has the effect of no longer requiring a
>   * KVA for addresses mapped into the S2. The presence of these features
>   * are thus necessary to support cacheable S2 mapping of VM_PFNMAP.
>   */
>  static inline bool kvm_supports_cacheable_pfnmap(void)
>  {
>  	return cpus_have_final_cap(ARM64_HAS_STAGE2_FWB) &&
>  	       cpus_have_final_cap(ARM64_HAS_CACHE_DIC);
>  }
>  
> +static inline bool kvm_supports_hafdbs(struct kvm *kvm)
> +{
> +	return IS_ENABLED(CONFIG_ARM64_HW_AFDBM) &&

cpucap_is_possible() is the right place to park this.

> 						      has_vhe() &&

I don't see a reason why this needs to be constrained to VHE-only.

> +		!kvm_vcpu_has_nv(kvm) && cpus_have_final_cap(ARM64_HW_DBM);

Same thing goes for nested... KVM can make use of HAFDBS in the
canonical stage-2 MMU (or even a shadow stage-2) independent of the
guest hypervisor.

>  #ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
>  void kvm_s2_ptdump_create_debugfs(struct kvm *kvm);
>  void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu);
>  void kvm_nested_s2_ptdump_remove_debugfs(struct kvm_s2_mmu *mmu);
>  #else
>  static inline void kvm_s2_ptdump_create_debugfs(struct kvm *kvm) {}
>  static inline void kvm_nested_s2_ptdump_create_debugfs(struct kvm_s2_mmu *mmu) {}
>  static inline void kvm_nested_s2_ptdump_remove_debugfs(struct kvm_s2_mmu *mmu) {}
>  #endif /* CONFIG_PTDUMP_STAGE2_DEBUGFS */
>  
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index 1ed708335809..9242b5d665af 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -1,24 +1,29 @@
>  /* SPDX-License-Identifier: GPL-2.0 */
>  #ifndef __ARM64_KVM_NESTED_H
>  #define __ARM64_KVM_NESTED_H
>  
>  #include <linux/bitfield.h>
>  #include <linux/kvm_host.h>
>  #include <asm/kvm_emulate.h>
>  #include <asm/kvm_pgtable.h>
>  
> -static inline bool vcpu_has_nv(const struct kvm_vcpu *vcpu)
> +static inline bool kvm_vcpu_has_nv(const struct kvm *kvm)

The name would suggest this thing takes a vcpu pointer...

Thanks,
Oliver