Re: [PATCH v2 12/16] KVM: arm64: nv: Tag shadow S2 entries with guest's leaf S2 level

[Oliver Upton <oliver.upton@linux.dev>]

On Wed, May 29, 2024 at 03:56:24PM +0100, Marc Zyngier wrote:
> Populate bits [56:55] of the leaf entry with the level provided
> by the guest's S2 translation. This will allow us to better scope
> the invalidation by remembering the mapping size.
> 
> Of course, this assume that the guest will issue an invalidation
> with an address that falls into the same leaf. If the guest doesn't,
> we'll over-invalidate.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/include/asm/kvm_nested.h |  8 ++++++++
>  arch/arm64/kvm/mmu.c                | 17 +++++++++++++++--
>  2 files changed, 23 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_nested.h b/arch/arm64/include/asm/kvm_nested.h
> index fcb0de3a93fe..971dbe533730 100644
> --- a/arch/arm64/include/asm/kvm_nested.h
> +++ b/arch/arm64/include/asm/kvm_nested.h
> @@ -5,6 +5,7 @@
>  #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)
>  {
> @@ -195,4 +196,11 @@ static inline bool kvm_auth_eretax(struct kvm_vcpu *vcpu, u64 *elr)
>  }
>  #endif
>  
> +#define KVM_NV_GUEST_MAP_SZ	(KVM_PGTABLE_PROT_SW1 | KVM_PGTABLE_PROT_SW0)
> +
> +static inline u64 kvm_encode_nested_level(struct kvm_s2_trans *trans)
> +{
> +	return FIELD_PREP(KVM_NV_GUEST_MAP_SZ, trans->level);
> +}
> +

It might be nice to keep all of the software fields for (in)valid in
a central place so we can add some documentation. I fear this is going
to get rather complicated as more pieces of pKVM land upstream and we
find new and fun ways to cram data into stage-2.

>  #endif /* __ARM64_KVM_NESTED_H */
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 4ed93a384255..f3a8ec70bd29 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -1598,11 +1598,17 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>  	 * Potentially reduce shadow S2 permissions to match the guest's own
>  	 * S2. For exec faults, we'd only reach this point if the guest
>  	 * actually allowed it (see kvm_s2_handle_perm_fault).
> +	 *
> +	 * Also encode the level of the nested translation in the SW bits of
> +	 * the PTE/PMD/PUD. This will be retrived on TLB invalidation from
> +	 * the guest.

typo: retrieved

Also, it might be helpful to add some color here to indicate the encoded
TTL is used to represent the span of a single virtual TLB entry,
providing scope to the TLBI by address.

Before I actually read what was going on, I thought the TTL in the PTE
was used for matching invalidation scopes that have a valid TTL.

-- 
Thanks,
Oliver