Subject: Re: [PATCH v6 42/44] KVM: VMX: Dedup code for adding MSR to VMCS's auto list

From: Sean Christopherson <[email-masked]>
Date: Fri, 20 Feb 2026 08:46:07 -0800

On Thu, Feb 19, 2026, Namhyung Kim wrote:
> Hello,
> 
> On Fri, Dec 05, 2025 at 04:17:18PM -0800, Sean Christopherson wrote:
> > Add a helper to add an MSR to a VMCS's "auto" list to deduplicate the code
> > in add_atomic_switch_msr(), and so that the functionality can be used in
> > the future for managing the MSR auto-store list.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Sean Christopherson <[email-masked]>
> > ---
> >  arch/x86/kvm/vmx/vmx.c | 41 +++++++++++++++++++----------------------
> >  1 file changed, 19 insertions(+), 22 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index 018e01daab68..3f64d4b1b19c 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -1093,12 +1093,28 @@ static __always_inline void add_atomic_switch_msr_special(struct vcpu_vmx *vmx,
> >  	vm_exit_controls_setbit(vmx, exit);
> >  }
> >  
> > +static void vmx_add_auto_msr(struct vmx_msrs *m, u32 msr, u64 value,
> > +			     unsigned long vmcs_count_field, struct kvm *kvm)
> > +{
> > +	int i;
> > +
> > +	i = vmx_find_loadstore_msr_slot(m, msr);
> > +	if (i < 0) {
> > +		if (KVM_BUG_ON(m->nr == MAX_NR_LOADSTORE_MSRS, kvm))
> > +			return;
> > +
> > +		i = m->nr++;
> > +		m->val[i].index = msr;
> > +		vmcs_write32(vmcs_count_field, m->nr);
> > +	}
> > +	m->val[i].value = value;
> > +}
> > +
> >  static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
> >  				  u64 guest_val, u64 host_val)
> >  {
> >  	struct msr_autoload *m = &vmx->msr_autoload;
> >  	struct kvm *kvm = vmx->vcpu.kvm;
> > -	int i;
> >  
> >  	switch (msr) {
> >  	case MSR_EFER:
> > @@ -1132,27 +1148,8 @@ static void add_atomic_switch_msr(struct vcpu_vmx *vmx, unsigned msr,
> >  		wrmsrq(MSR_IA32_PEBS_ENABLE, 0);
> >  	}
> >  
> > -	i = vmx_find_loadstore_msr_slot(&m->guest, msr);
> > -	if (i < 0) {
> > -		if (KVM_BUG_ON(m->guest.nr == MAX_NR_LOADSTORE_MSRS, kvm))
> > -			return;
> > -
> > -		i = m->guest.nr++;
> > -		m->guest.val[i].index = msr;
> > -		vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, m->guest.nr);
> > -	}
> > -	m->guest.val[i].value = guest_val;
> > -
> > -	i = vmx_find_loadstore_msr_slot(&m->host, msr);
> > -	if (i < 0) {
> > -		if (KVM_BUG_ON(m->host.nr == MAX_NR_LOADSTORE_MSRS, kvm))
> > -			return;
> > -
> > -		i = m->host.nr++;
> > -		m->host.val[i].index = msr;
> > -		vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, m->host.nr);
> > -	}
> > -	m->host.val[i].value = host_val;
> > +	vmx_add_auto_msr(&m->guest, msr, guest_val, VM_ENTRY_MSR_LOAD_COUNT, kvm);
> > +	vmx_add_auto_msr(&m->guest, msr, host_val, VM_EXIT_MSR_LOAD_COUNT, kvm);
> 
> Shouldn't it be &m->host for the host_val?

Ouch.  Yes.  How on earth did this escape testing...  Ah, because in practice
only MSR_IA32_PEBS_ENABLE goes through the load lists, and the VM-Entry load list
will use the guest's value due to VM_ENTRY_MSR_LOAD_COUNT not covering the bad
host value.

Did you happen to run into problems when using PEBS events in the host?

Regardless, do you want to send a patch?  Either way, I'll figure out a way to
verify the bug and the fix.