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

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Message-ID: <f551c638-a8c8-4aab-9fb7-f04df124ff15@linux.intel.com>
Date: Mon, 8 Dec 2025 17:32:01 +0800
From: "Mi, Dapeng" <dapeng1.mi@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>, 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>, 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>,
 Xiong Zhang <xiong.y.zhang@...ux.intel.com>,
 Manali Shukla <manali.shukla@....com>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v6 38/44] KVM: VMX: Drop unused @entry_only param from
 add_atomic_switch_msr()


On 12/6/2025 8:17 AM, Sean Christopherson wrote:
> 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)

LGTM.

Reviewed-by: Dapeng Mi <dapeng1.mi@...ux.intel.com>