Re: [PATCH 02/16] KVM: x86: Remove separate "bit" defines for page fault error code masks

From: Paolo Bonzini <pbonzini@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	 Yan Zhao <yan.y.zhao@intel.com>,
	Isaku Yamahata <isaku.yamahata@intel.com>,
	 Michael Roth <michael.roth@amd.com>,
	Yu Zhang <yu.c.zhang@linux.intel.com>,
	 Chao Peng <chao.p.peng@linux.intel.com>,
	Fuad Tabba <tabba@google.com>,
	 David Matlack <dmatlack@google.com>
Subject: Re: [PATCH 02/16] KVM: x86: Remove separate "bit" defines for page fault error code masks
Date: Thu, 29 Feb 2024 13:44:16 +0100	[thread overview]
Message-ID: <CABgObfbtPJ6AAX9GnjNscPRTbNAOtamdxX677kx_r=zd4scw6w@mail.gmail.com> (raw)
In-Reply-To: <20240228024147.41573-3-seanjc@google.com>

On Wed, Feb 28, 2024 at 3:46 AM Sean Christopherson <seanjc@google.com> wrote:
>
> Open code the bit number directly in the PFERR_* masks and drop the
> intermediate PFERR_*_BIT defines, as having to bounce through two macros
> just to see which flag corresponds to which bit is quite annoying, as is
> having to define two macros just to add recognition of a new flag.
>
> Use ilog2() to derive the bit in permission_fault(), the one function that
> actually needs the bit number (it does clever shifting to manipulate flags
> in order to avoid conditional branches).
>
> No functional change intended.
>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 32 ++++++++++----------------------
>  arch/x86/kvm/mmu.h              |  4 ++--
>  2 files changed, 12 insertions(+), 24 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index aaf5a25ea7ed..88cc523bafa8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -254,28 +254,16 @@ enum x86_intercept_stage;
>         KVM_GUESTDBG_INJECT_DB | \
>         KVM_GUESTDBG_BLOCKIRQ)
>
> -
> -#define PFERR_PRESENT_BIT 0
> -#define PFERR_WRITE_BIT 1
> -#define PFERR_USER_BIT 2
> -#define PFERR_RSVD_BIT 3
> -#define PFERR_FETCH_BIT 4
> -#define PFERR_PK_BIT 5
> -#define PFERR_SGX_BIT 15
> -#define PFERR_GUEST_FINAL_BIT 32
> -#define PFERR_GUEST_PAGE_BIT 33
> -#define PFERR_IMPLICIT_ACCESS_BIT 48
> -
> -#define PFERR_PRESENT_MASK     BIT(PFERR_PRESENT_BIT)
> -#define PFERR_WRITE_MASK       BIT(PFERR_WRITE_BIT)
> -#define PFERR_USER_MASK                BIT(PFERR_USER_BIT)
> -#define PFERR_RSVD_MASK                BIT(PFERR_RSVD_BIT)
> -#define PFERR_FETCH_MASK       BIT(PFERR_FETCH_BIT)
> -#define PFERR_PK_MASK          BIT(PFERR_PK_BIT)
> -#define PFERR_SGX_MASK         BIT(PFERR_SGX_BIT)
> -#define PFERR_GUEST_FINAL_MASK BIT_ULL(PFERR_GUEST_FINAL_BIT)
> -#define PFERR_GUEST_PAGE_MASK  BIT_ULL(PFERR_GUEST_PAGE_BIT)
> -#define PFERR_IMPLICIT_ACCESS  BIT_ULL(PFERR_IMPLICIT_ACCESS_BIT)
> +#define PFERR_PRESENT_MASK     BIT(0)
> +#define PFERR_WRITE_MASK       BIT(1)
> +#define PFERR_USER_MASK                BIT(2)
> +#define PFERR_RSVD_MASK                BIT(3)
> +#define PFERR_FETCH_MASK       BIT(4)
> +#define PFERR_PK_MASK          BIT(5)
> +#define PFERR_SGX_MASK         BIT(15)
> +#define PFERR_GUEST_FINAL_MASK BIT_ULL(32)
> +#define PFERR_GUEST_PAGE_MASK  BIT_ULL(33)
> +#define PFERR_IMPLICIT_ACCESS  BIT_ULL(48)
>
>  #define PFERR_NESTED_GUEST_PAGE (PFERR_GUEST_PAGE_MASK |       \
>                                  PFERR_WRITE_MASK |             \
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 60f21bb4c27b..e8b620a85627 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -213,7 +213,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>          */
>         u64 implicit_access = access & PFERR_IMPLICIT_ACCESS;
>         bool not_smap = ((rflags & X86_EFLAGS_AC) | implicit_access) == X86_EFLAGS_AC;
> -       int index = (pfec + (not_smap << PFERR_RSVD_BIT)) >> 1;
> +       int index = (pfec + (not_smap << ilog2(PFERR_RSVD_MASK))) >> 1;

Just use "(pfec + (not_smap ? PFERR_RSVD_MASK : 0)) >> 1".

Likewise below, "pte_access & PT_USER_MASK ? PFERR_RSVD_MASK : 0"/

No need to even check what the compiler produces, it will be either
exactly the same code or a bunch of cmov instructions.

Paolo

>         u32 errcode = PFERR_PRESENT_MASK;
>         bool fault;
>
> @@ -235,7 +235,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>
>                 /* clear present bit, replace PFEC.RSVD with ACC_USER_MASK. */
>                 offset = (pfec & ~1) +
> -                       ((pte_access & PT_USER_MASK) << (PFERR_RSVD_BIT - PT_USER_SHIFT));
> +                       ((pte_access & PT_USER_MASK) << (ilog2(PFERR_RSVD_MASK) - PT_USER_SHIFT));
>
>                 pkru_bits &= mmu->pkru_mask >> offset;
>                 errcode |= -pkru_bits & PFERR_PK_MASK;
> --
> 2.44.0.278.ge034bb2e1d-goog
>