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    SubjectRe: [PATCH v2 14/18] KVM: x86/mmu: avoid indirect call for get_cr3
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    On Thu, 2022-02-17 at 16:03 -0500, Paolo Bonzini wrote:
    > Most of the time, calls to get_guest_pgd result in calling
    > kvm_read_cr3 (the exception is only nested TDP). Hardcode
    > the default instead of using the get_cr3 function, avoiding
    > a retpoline if they are enabled.
    >
    > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    > ---
    > arch/x86/kvm/mmu.h | 13 +++++++++++++
    > arch/x86/kvm/mmu/mmu.c | 15 +++++----------
    > arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
    > arch/x86/kvm/x86.c | 2 +-
    > 4 files changed, 20 insertions(+), 12 deletions(-)
    >
    > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
    > index 1d0c1904d69a..1808d6814ddb 100644
    > --- a/arch/x86/kvm/mmu.h
    > +++ b/arch/x86/kvm/mmu.h
    > @@ -116,6 +116,19 @@ static inline void kvm_mmu_load_pgd(struct kvm_vcpu *vcpu)
    > vcpu->arch.mmu->shadow_root_level);
    > }
    >
    > +static inline gpa_t __kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu)
    > +{
    > + if (!mmu->get_guest_pgd)
    > + return kvm_read_cr3(vcpu);
    > + else
    > + return mmu->get_guest_pgd(vcpu);
    > +}
    > +
    > +static inline gpa_t kvm_mmu_get_guest_pgd(struct kvm_vcpu *vcpu)
    > +{
    > + return __kvm_mmu_get_guest_pgd(vcpu, vcpu->arch.mmu);
    > +}
    > +
    > struct kvm_page_fault {
    > /* arguments to kvm_mmu_do_page_fault. */
    > const gpa_t addr;
    > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
    > index 4e8e3e9530ca..d422d0d2adf8 100644
    > --- a/arch/x86/kvm/mmu/mmu.c
    > +++ b/arch/x86/kvm/mmu/mmu.c
    > @@ -3451,7 +3451,7 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
    > unsigned i;
    > int r;
    >
    > - root_pgd = mmu->get_guest_pgd(vcpu);
    > + root_pgd = kvm_mmu_get_guest_pgd(vcpu);
    > root_gfn = root_pgd >> PAGE_SHIFT;
    >
    > if (mmu_check_root(vcpu, root_gfn))
    > @@ -3881,7 +3881,7 @@ static bool kvm_arch_setup_async_pf(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
    > arch.token = (vcpu->arch.apf.id++ << 12) | vcpu->vcpu_id;
    > arch.gfn = gfn;
    > arch.direct_map = vcpu->arch.mmu->direct_map;
    > - arch.cr3 = vcpu->arch.mmu->get_guest_pgd(vcpu);
    > + arch.cr3 = kvm_mmu_get_guest_pgd(vcpu);
    >
    > return kvm_setup_async_pf(vcpu, cr2_or_gpa,
    > kvm_vcpu_gfn_to_hva(vcpu, gfn), &arch);
    > @@ -4230,11 +4230,6 @@ void kvm_mmu_new_pgd(struct kvm_vcpu *vcpu, gpa_t new_pgd)
    > }
    > EXPORT_SYMBOL_GPL(kvm_mmu_new_pgd);
    >
    > -static unsigned long get_cr3(struct kvm_vcpu *vcpu)
    > -{
    > - return kvm_read_cr3(vcpu);
    > -}
    > -
    > static bool sync_mmio_spte(struct kvm_vcpu *vcpu, u64 *sptep, gfn_t gfn,
    > unsigned int access)
    > {
    > @@ -4789,7 +4784,7 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
    > context->invlpg = NULL;
    > context->shadow_root_level = kvm_mmu_get_tdp_level(vcpu);
    > context->direct_map = true;
    > - context->get_guest_pgd = get_cr3;
    > + context->get_guest_pgd = NULL; /* use kvm_read_cr3 */
    > context->get_pdptr = kvm_pdptr_read;
    > context->inject_page_fault = kvm_inject_page_fault;
    > context->root_level = role_regs_to_root_level(&regs);
    > @@ -4964,7 +4959,7 @@ static void init_kvm_softmmu(struct kvm_vcpu *vcpu)
    >
    > kvm_init_shadow_mmu(vcpu, &regs);
    >
    > - context->get_guest_pgd = get_cr3;
    > + context->get_guest_pgd = NULL; /* use kvm_read_cr3 */
    > context->get_pdptr = kvm_pdptr_read;
    > context->inject_page_fault = kvm_inject_page_fault;
    > }
    > @@ -4996,7 +4991,7 @@ static void init_kvm_nested_mmu(struct kvm_vcpu *vcpu)
    > return;
    >
    > g_context->mmu_role.as_u64 = new_role.as_u64;
    > - g_context->get_guest_pgd = get_cr3;
    > + g_context->get_guest_pgd = NULL; /* use kvm_read_cr3 */
    > g_context->get_pdptr = kvm_pdptr_read;
    > g_context->inject_page_fault = kvm_inject_page_fault;
    > g_context->root_level = new_role.base.level;
    > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
    > index 346f3bad3cb9..1a85aba837b2 100644
    > --- a/arch/x86/kvm/mmu/paging_tmpl.h
    > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
    > @@ -362,7 +362,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
    > trace_kvm_mmu_pagetable_walk(addr, access);
    > retry_walk:
    > walker->level = mmu->root_level;
    > - pte = mmu->get_guest_pgd(vcpu);
    > + pte = __kvm_mmu_get_guest_pgd(vcpu, mmu);
    > have_ad = PT_HAVE_ACCESSED_DIRTY(mmu);
    >
    > #if PTTYPE == 64
    > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
    > index f10878aa5b20..adcee7c305ca 100644
    > --- a/arch/x86/kvm/x86.c
    > +++ b/arch/x86/kvm/x86.c
    > @@ -12161,7 +12161,7 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
    > return;
    >
    > if (!vcpu->arch.mmu->direct_map &&
    > - work->arch.cr3 != vcpu->arch.mmu->get_guest_pgd(vcpu))
    > + work->arch.cr3 != kvm_mmu_get_guest_pgd(vcpu))
    > return;
    >
    > kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true);


    Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

    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.

    Best regards,
    Maxim Levitsky



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