Re: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3

Maxim Levitsky
2022-02-24T15:15:08Z

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);
>