linux-kernel - Re: [PATCH 02/17] KVM: x86: Remove separate "bit" defines for page fault error code masks

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Date: Mon, 13 May 2024 13:29:02 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
 kvm@...r.kernel.org
Cc: Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 02/17] KVM: x86: Remove separate "bit" defines for page
 fault error code masks

On 5/7/2024 11:58 PM, Paolo Bonzini wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> 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,

can't agree more on it.

> as is
> having to define two macros just to add recognition of a new flag.
> 
> Use ternary operator to derive the bit in permission_fault(), the one
> function that actually needs the bit number as part of clever shifting
> to avoid conditional branches.  Generally the compiler is able to turn
> it into a conditional move, and if not it's not really a big deal.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> Message-ID: <20240228024147.41573-3-seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.co
Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

> ---
>   arch/x86/include/asm/kvm_host.h | 32 ++++++++++----------------------
>   arch/x86/kvm/mmu.h              |  5 ++---
>   2 files changed, 12 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 9f92bdb78504..a047480da5af 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..2343c9f00e31 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 ? PFERR_RSVD_MASK : 0)) >> 1;
>   	u32 errcode = PFERR_PRESENT_MASK;
>   	bool fault;
>   
> @@ -234,8 +234,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
>   		pkru_bits = (vcpu->arch.pkru >> (pte_pkey * 2)) & 3;
>   
>   		/* clear present bit, replace PFEC.RSVD with ACC_USER_MASK. */
> -		offset = (pfec & ~1) +
> -			((pte_access & PT_USER_MASK) << (PFERR_RSVD_BIT - PT_USER_SHIFT));
> +		offset = (pfec & ~1) | ((pte_access & PT_USER_MASK) ? PFERR_RSVD_MASK : 0);
>   
>   		pkru_bits &= mmu->pkru_mask >> offset;
>   		errcode |= -pkru_bits & PFERR_PK_MASK;