linux-kernel - Re: [Patch v2 1/5] KVM: x86/mmu: Make separate function to check for SPTEs atomic write conditions

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Date:   Mon, 6 Feb 2023 14:09:42 -0800
From:   Ben Gardon <bgardon@...gle.com>
To:     Vipin Sharma <vipinsh@...gle.com>
Cc:     seanjc@...gle.com, pbonzini@...hat.com, dmatlack@...gle.com,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [Patch v2 1/5] KVM: x86/mmu: Make separate function to check for
 SPTEs atomic write conditions

On Fri, Feb 3, 2023 at 11:28 AM Vipin Sharma <vipinsh@...gle.com> wrote:
>
> Move condition checks in kvm_tdp_mmu_write_spte() for writing spte
> atomically in a separate function.
>
> New function will be used in future commits to clear bits in SPTE.
>
> Signed-off-by: Vipin Sharma <vipinsh@...gle.com>

Reviewed-by: Ben Gardon <bgardon@...gle.com>

> ---
>  arch/x86/kvm/mmu/tdp_iter.h | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/tdp_iter.h b/arch/x86/kvm/mmu/tdp_iter.h
> index f0af385c56e0..30a52e5e68de 100644
> --- a/arch/x86/kvm/mmu/tdp_iter.h
> +++ b/arch/x86/kvm/mmu/tdp_iter.h
> @@ -29,11 +29,10 @@ static inline void __kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 new_spte)
>         WRITE_ONCE(*rcu_dereference(sptep), new_spte);
>  }
>
> -static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> -                                        u64 new_spte, int level)
> +static inline bool kvm_tdp_mmu_spte_has_volatile_bits(u64 old_spte, int level)
>  {
>         /*
> -        * Atomically write the SPTE if it is a shadow-present, leaf SPTE with
> +        * Atomically write SPTEs if it is a shadow-present, leaf SPTE with

Nit: SPTEs must be modified atomically if they are shadow-present,
leaf SPTEs with

>          * volatile bits, i.e. has bits that can be set outside of mmu_lock.
>          * The Writable bit can be set by KVM's fast page fault handler, and
>          * Accessed and Dirty bits can be set by the CPU.
> @@ -44,8 +43,15 @@ static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
>          * logic needs to be reassessed if KVM were to use non-leaf Accessed
>          * bits, e.g. to skip stepping down into child SPTEs when aging SPTEs.
>          */
> -       if (is_shadow_present_pte(old_spte) && is_last_spte(old_spte, level) &&
> -           spte_has_volatile_bits(old_spte))
> +       return is_shadow_present_pte(old_spte) &&
> +              is_last_spte(old_spte, level) &&
> +              spte_has_volatile_bits(old_spte);
> +}
> +
> +static inline u64 kvm_tdp_mmu_write_spte(tdp_ptep_t sptep, u64 old_spte,
> +                                        u64 new_spte, int level)
> +{
> +       if (kvm_tdp_mmu_spte_has_volatile_bits(old_spte, level))
>                 return kvm_tdp_mmu_write_spte_atomic(sptep, new_spte);
>
>         __kvm_tdp_mmu_write_spte(sptep, new_spte);
> --
> 2.39.1.519.gcb327c4b5f-goog
>