linux-kernel - [PATCH] KVM: x86/mmu: rename trace function name for asynchronous page fault

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Message-Id: <20220807052141.69186-1-mizhang@google.com>
Date:   Sun,  7 Aug 2022 05:21:41 +0000
From:   Mingwei Zhang <mizhang@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mingwei Zhang <mizhang@...gle.com>
Subject: [PATCH] KVM: x86/mmu: rename trace function name for asynchronous
 page fault

Rename the tracepoint function from trace_kvm_async_pf_doublefault() to
trace_kvm_async_pf_repeated_fault() to make it clear, since double fault
has nothing to do with this trace function.

Asynchronous Page Fault (APF) is an artifact generated by KVM when it
cannot find a physical page to satisfy an EPT violation. KVM uses APF to
tell the guest OS to do something else such as scheduling other guest
processes to make forward progress. However, when another guest process
also touches a previously APFed page, KVM halts the vCPU instead of
generating a repeated APF to avoid wasting cycles.

Double fault (#DF) clearly has a different meaning and a different
consequence when triggered. #DF requires two nested contributory exceptions
instead of two page faults faulting at the same address. A prevous bug on
APF indicates that it may trigger a double fault in the guest [1] and
clearly this trace function has nothing to do with it. So rename this
function should be a valid choice.

No functional change intended.

[1] https://www.spinics.net/lists/kvm/msg214957.html

Signed-off-by: Mingwei Zhang <mizhang@...gle.com>
---
 arch/x86/kvm/mmu/mmu.c     | 2 +-
 include/trace/events/kvm.h | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
index 06ac8c7cef67..0004ec68fc5c 100644
--- a/arch/x86/kvm/mmu/mmu.c
+++ b/arch/x86/kvm/mmu/mmu.c
@@ -4164,7 +4164,7 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
 	if (!fault->prefetch && kvm_can_do_async_pf(vcpu)) {
 		trace_kvm_try_async_get_page(fault->addr, fault->gfn);
 		if (kvm_find_async_pf_gfn(vcpu, fault->gfn)) {
-			trace_kvm_async_pf_doublefault(fault->addr, fault->gfn);
+			trace_kvm_async_pf_repeated_fault(fault->addr, fault->gfn);
 			kvm_make_request(KVM_REQ_APF_HALT, vcpu);
 			return RET_PF_RETRY;
 		} else if (kvm_arch_setup_async_pf(vcpu, fault->addr, fault->gfn)) {
diff --git a/include/trace/events/kvm.h b/include/trace/events/kvm.h
index 37e1e1a2d67d..3bd31ea23fee 100644
--- a/include/trace/events/kvm.h
+++ b/include/trace/events/kvm.h
@@ -282,7 +282,7 @@ DEFINE_EVENT(kvm_async_get_page_class, kvm_try_async_get_page,
 	TP_ARGS(gva, gfn)
 );

-DEFINE_EVENT(kvm_async_get_page_class, kvm_async_pf_doublefault,
+DEFINE_EVENT(kvm_async_get_page_class, kvm_async_pf_repeated_fault,

 	TP_PROTO(u64 gva, u64 gfn),

base-commit: 1612c382ffbdf1f673caec76502b1c00e6d35363
-- 
2.37.1.559.g78731f0fdb-goog