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Subject[PATCH] KVM: x86/mmu: Avoid retpoline on ->page_fault() with TDP
Date
Wrap calls to ->page_fault() with a small shim to directly invoke the
TDP fault handler when the kernel is using retpolines and TDP is being
used. Denote the TDP fault handler by nullifying mmu->page_fault, and
annotate the TDP path as likely to coerce the compiler into preferring
the TDP path.

Rename tdp_page_fault() to kvm_tdp_page_fault() as it's exposed outside
of mmu.c to allow inlining the shim.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
---

Haven't done any performance testing, this popped into my head when mucking
with the 5-level page table crud as an easy way to shave cycles in the
happy path.

arch/x86/kvm/mmu.h | 13 +++++++++++++
arch/x86/kvm/mmu/mmu.c | 16 ++++++++++------
arch/x86/kvm/x86.c | 2 +-
3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
index d55674f44a18..9277ee8a54a5 100644
--- a/arch/x86/kvm/mmu.h
+++ b/arch/x86/kvm/mmu.h
@@ -102,6 +102,19 @@ static inline void kvm_mmu_load_cr3(struct kvm_vcpu *vcpu)
kvm_get_active_pcid(vcpu));
}

+int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
+ bool prefault);
+
+static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
+ u32 err, bool prefault)
+{
+#ifdef CONFIG_RETPOLINE
+ if (likely(!vcpu->arch.mmu->page_fault))
+ return kvm_tdp_page_fault(vcpu, cr2_or_gpa, err, prefault);
+#endif
+ return vcpu->arch.mmu->page_fault(vcpu, cr2_or_gpa, err, prefault);
+}
+
/*
* Currently, we have two sorts of write-protection, a) the first one
* write-protects guest page to sync the guest modification, b) another one is
diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 7011a4e54866..5267f1440677 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4219,8 +4219,8 @@ int kvm_handle_page_fault(struct kvm_vcpu *vcpu, u64 error_code,
}
EXPORT_SYMBOL_GPL(kvm_handle_page_fault);

-static int tdp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
- bool prefault)
+int kvm_tdp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
+ bool prefault)
{
int max_level;

@@ -4925,7 +4925,12 @@ static void init_kvm_tdp_mmu(struct kvm_vcpu *vcpu)
return;

context->mmu_role.as_u64 = new_role.as_u64;
- context->page_fault = tdp_page_fault;
+#ifdef CONFIG_RETPOLINE
+ /* Nullify ->page_fault() to use direct kvm_tdp_page_fault() call. */
+ context->page_fault = NULL;
+#else
+ context->page_fault = kvm_tdp_page_fault;
+#endif
context->sync_page = nonpaging_sync_page;
context->invlpg = nonpaging_invlpg;
context->update_pte = nonpaging_update_pte;
@@ -5436,9 +5441,8 @@ int kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 error_code,
}

if (r == RET_PF_INVALID) {
- r = vcpu->arch.mmu->page_fault(vcpu, cr2_or_gpa,
- lower_32_bits(error_code),
- false);
+ r = kvm_mmu_do_page_fault(vcpu, cr2_or_gpa,
+ lower_32_bits(error_code), false);
WARN_ON(r == RET_PF_INVALID);
}

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index fbabb2f06273..39251ecafd2b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10182,7 +10182,7 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
work->arch.cr3 != vcpu->arch.mmu->get_cr3(vcpu))
return;

- vcpu->arch.mmu->page_fault(vcpu, work->cr2_or_gpa, 0, true);
+ kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true);
}

static inline u32 kvm_async_pf_hash_fn(gfn_t gfn)
--
2.24.1
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