linux-kernel - Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get

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Message-ID: <cbc04a96e59c86a1ad77dcbdc10fabfb45066c89.camel@redhat.com>
Date:   Thu, 24 Feb 2022 17:14:53 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3

On Thu, 2022-02-24 at 15:12 +0000, Sean Christopherson wrote:
> On Thu, Feb 24, 2022, Maxim Levitsky wrote:
> > Not sure though if that is worth it though. IMHO it would be better to
> > convert mmu callbacks (and nested ops callbacks, etc) to static calls.
> 
> nested_ops can utilize static_call(), mmu hooks cannot.  static_call() patches
> the code, which means there cannot be multiple targets at any given time.  The
> "static" part refers to the target not changing, generally for the lifetime of
> the kernel/module in question.  Even with TDP that doesn't hold true due to
> nested virtualization.

Ah, right, I forgot that static_call patches the call sites.

Best regards,
	Maxim Levitsky

> 
> We could selectively use INDIRECT_CALL_*() for some of the MMU calls, but given
> how few cases and targets we really care about, I prefer our homebrewed manual
> checks as theres less macro maze to navigate.
> 
> E.g. to convert the TDP fault case
> 
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 1d0c1904d69a..940ec6a9d284 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -3,6 +3,8 @@
>  #define __KVM_X86_MMU_H
> 
>  #include <linux/kvm_host.h>
> +#include <linux/indirect_call_wrapper.h>
> +
>  #include "kvm_cache_regs.h"
>  #include "cpuid.h"
> 
> @@ -169,7 +171,8 @@ struct kvm_page_fault {
>         bool map_writable;
>  };
> 
> -int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
> +INDIRECT_CALLABLE_DECLARE(int kvm_tdp_page_fault(struct kvm_vcpu *vcpu,
> +                                                struct kvm_page_fault *fault));
> 
>  extern int nx_huge_pages;
>  static inline bool is_nx_huge_page_enabled(void)
> @@ -196,11 +199,9 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>                 .req_level = PG_LEVEL_4K,
>                 .goal_level = PG_LEVEL_4K,
>         };
> -#ifdef CONFIG_RETPOLINE
> -       if (fault.is_tdp)
> -               return kvm_tdp_page_fault(vcpu, &fault);
> -#endif
> -       return vcpu->arch.mmu->page_fault(vcpu, &fault);
> +       struct kvm_mmu *mmu = vcpu->arch.mmu;
> +
> +       return INDIRECT_CALL_1(mmu->page_fault, kvm_tdp_page_fault, vcpu, &fault);
>  }
> 
>  /*
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index c1deaec795c2..a3ad1bc58859 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -4055,7 +4055,8 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
>  }
>  EXPORT_SYMBOL_GPL(kvm_handle_page_fault);
> 
> -int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> +INDIRECT_CALLABLE_SCOPE int kvm_tdp_page_fault(struct kvm_vcpu *vcpu,
> +                                              struct kvm_page_fault *fault)
>  {
>         while (fault->max_level > PG_LEVEL_4K) {
>                 int page_num = KVM_PAGES_PER_HPAGE(fault->max_level);
>