linux-kernel - [PATCH v6 38/44] KVM: VMX: Drop unused @entry_only param from add_atomic_switch

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Message-ID: <20251206001720.468579-39-seanjc@google.com>
Date: Fri,  5 Dec 2025 16:17:14 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Marc Zyngier <maz@...nel.org>, Oliver Upton <oupton@...nel.org>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Huacai Chen <chenhuacai@...nel.org>, Anup Patel <anup@...infault.org>, 
	Paul Walmsley <pjw@...nel.org>, Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Xin Li <xin@...or.com>, "H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, Namhyung Kim <namhyung@...nel.org>, 
	Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, loongarch@...ts.linux.dev, kvm-riscv@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org, Mingwei Zhang <mizhang@...gle.com>, 
	Xudong Hao <xudong.hao@...el.com>, Sandipan Das <sandipan.das@....com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Xiong Zhang <xiong.y.zhang@...ux.intel.com>, 
	Manali Shukla <manali.shukla@....com>, Jim Mattson <jmattson@...gle.com>
Subject: [PATCH v6 38/44] KVM: VMX: Drop unused @entry_only param from add_atomic_switch_msr()

Drop the "on VM-Enter only" parameter from add_atomic_switch_msr() as it
is no longer used, and for all intents and purposes was never used.  The
functionality was added, under embargo, by commit 989e3992d2ec
("x86/KVM/VMX: Extend add_atomic_switch_msr() to allow VMENTER only MSRs"),
and then ripped out by commit 2f055947ae5e ("x86/kvm: Drop L1TF MSR list
approach") just a few commits later.

  2f055947ae5e x86/kvm: Drop L1TF MSR list approach
  72c6d2db64fa x86/litf: Introduce vmx status variable
  215af5499d9e cpu/hotplug: Online siblings when SMT control is turned on
  390d975e0c4e x86/KVM/VMX: Use MSR save list for IA32_FLUSH_CMD if required
  989e3992d2ec x86/KVM/VMX: Extend add_atomic_switch_msr() to allow VMENTER only MSRs

Furthermore, it's extremely unlikely KVM will ever _need_ to load an MSR
value via the auto-load lists only on VM-Enter.  MSRs writes via the lists
aren't optimized in any way, and so the only reason to use the lists
instead of a WRMSR are for cases where the MSR _must_ be load atomically
with respect to VM-Enter (and/or VM-Exit).  While one could argue that
command MSRs, e.g. IA32_FLUSH_CMD, "need" to be done exact at VM-Enter, in
practice doing such flushes within a few instructons of VM-Enter is more
than sufficient.

Note, the shortlog and changelog for commit 390d975e0c4e ("x86/KVM/VMX: Use
MSR save list for IA32_FLUSH_CMD if required") are misleading and wrong.
That commit added MSR_IA32_FLUSH_CMD to the VM-Enter _load_ list, not the
VM-Enter save list (which doesn't exist, only VM-Exit has a store/save
list).

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/vmx/vmx.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a51f66d1b201..38491962b2c1 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -1094,7 +1094,7 @@ static __always_inline void add_atomic_switch_msr_special(struct vcpu_vmx *vmx,
 }
 
 static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
-				  u64 guest_val, u64 host_val, bool entry_only)
+				  u64 guest_val, u64 host_val)
 {
 	int i, j = 0;
 	struct msr_autoload *m = &vmx->msr_autoload;
@@ -1132,8 +1132,7 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
 	}
 
 	i = vmx_find_loadstore_msr_slot(&m->guest, msr);
-	if (!entry_only)
-		j = vmx_find_loadstore_msr_slot(&m->host, msr);
+	j = vmx_find_loadstore_msr_slot(&m->host, msr);
 
 	if ((i < 0 && m->guest.nr == MAX_NR_LOADSTORE_MSRS) ||
 	    (j < 0 &&  m->host.nr == MAX_NR_LOADSTORE_MSRS)) {
@@ -1148,9 +1147,6 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
 	m->guest.val[i].index = msr;
 	m->guest.val[i].value = guest_val;
 
-	if (entry_only)
-		return;
-
 	if (j < 0) {
 		j = m->host.nr++;
 		vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, m->host.nr);
@@ -1190,8 +1186,7 @@ static bool update_transition_efer(struct vcpu_vmx *vmx)
 		if (!(guest_efer & EFER_LMA))
 			guest_efer &= ~EFER_LME;
 		if (guest_efer != kvm_host.efer)
-			add_atomic_switch_msr(vmx, MSR_EFER,
-					      guest_efer, kvm_host.efer, false);
+			add_atomic_switch_msr(vmx, MSR_EFER, guest_efer, kvm_host.efer);
 		else
 			clear_atomic_switch_msr(vmx, MSR_EFER);
 		return false;
@@ -7350,7 +7345,7 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx)
 			clear_atomic_switch_msr(vmx, msrs[i].msr);
 		else
 			add_atomic_switch_msr(vmx, msrs[i].msr, msrs[i].guest,
-					msrs[i].host, false);
+					      msrs[i].host);
 }
 
 static void vmx_update_hv_timer(struct kvm_vcpu *vcpu, bool force_immediate_exit)
-- 
2.52.0.223.gf5cc29aaa4-goog