Re: [PATCH] KVM: x86: Restrict get_mt_mask() to a u8, use KVM_X86_OP_OPTIONAL_RET0

[Paolo Bonzini <pbonzini@redhat.com>]

On 7/14/22 17:37, Sean Christopherson wrote:
> Restrict get_mt_mask() to a u8 and reintroduce using a RET0 static_call
> for the SVM implementation.  EPT stores the memtype information in the
> lower 8 bits (bits 6:3 to be precise), and even returns a shifted u8
> without an explicit cast to a larger type; there's no need to return a
> full u64.
> 
> Note, RET0 doesn't play nice with a u64 return on 32-bit kernels, see
> commit bf07be36cd88 ("KVM: x86: do not use KVM_X86_OP_OPTIONAL_RET0 for
> get_mt_mask").
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Doh, that's obvious.  Queued, thanks.

Paolo

> ---
>   arch/x86/include/asm/kvm-x86-ops.h | 2 +-
>   arch/x86/include/asm/kvm_host.h    | 2 +-
>   arch/x86/kvm/svm/svm.c             | 6 ------
>   arch/x86/kvm/vmx/vmx.c             | 2 +-
>   4 files changed, 3 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 6f2f1affbb78..51f777071584 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -88,7 +88,7 @@ KVM_X86_OP(deliver_interrupt)
>   KVM_X86_OP_OPTIONAL(sync_pir_to_irr)
>   KVM_X86_OP_OPTIONAL_RET0(set_tss_addr)
>   KVM_X86_OP_OPTIONAL_RET0(set_identity_map_addr)
> -KVM_X86_OP(get_mt_mask)
> +KVM_X86_OP_OPTIONAL_RET0(get_mt_mask)
>   KVM_X86_OP(load_mmu_pgd)
>   KVM_X86_OP(has_wbinvd_exit)
>   KVM_X86_OP(get_l2_tsc_offset)
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index de5a149d0971..fa4b2392fba0 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1546,7 +1546,7 @@ struct kvm_x86_ops {
>   	int (*sync_pir_to_irr)(struct kvm_vcpu *vcpu);
>   	int (*set_tss_addr)(struct kvm *kvm, unsigned int addr);
>   	int (*set_identity_map_addr)(struct kvm *kvm, u64 ident_addr);
> -	u64 (*get_mt_mask)(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio);
> +	u8 (*get_mt_mask)(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio);
>   
>   	void (*load_mmu_pgd)(struct kvm_vcpu *vcpu, hpa_t root_hpa,
>   			     int root_level);
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 37ce061dfc76..19af6dacfc5b 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4158,11 +4158,6 @@ static bool svm_has_emulated_msr(struct kvm *kvm, u32 index)
>   	return true;
>   }
>   
> -static u64 svm_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> -{
> -	return 0;
> -}
> -
>   static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>   {
>   	struct vcpu_svm *svm = to_svm(vcpu);
> @@ -4814,7 +4809,6 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>   	.check_apicv_inhibit_reasons = avic_check_apicv_inhibit_reasons,
>   	.apicv_post_state_restore = avic_apicv_post_state_restore,
>   
> -	.get_mt_mask = svm_get_mt_mask,
>   	.get_exit_info = svm_get_exit_info,
>   
>   	.vcpu_after_set_cpuid = svm_vcpu_after_set_cpuid,
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index c30115b9cb33..c895a3b6824d 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -7352,7 +7352,7 @@ static int __init vmx_check_processor_compat(void)
>   	return 0;
>   }
>   
> -static u64 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> +static u8 vmx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
>   {
>   	u8 cache;
>   
> 
> base-commit: b9b71f43683ae9d76b0989249607bbe8c9eb6c5c