[PATCH v6 38/44] KVM: VMX: Drop unused @entry_only param from add_atomic_switch_msr()

Sean Christopherson
2025-12-06T00:18:44Z

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@google.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