Re: [PATCH 2/3] KVM: x86: Don't update KVM PV feature CPUID during vCPU running

[Sean Christopherson]

+Kechen

On Thu, Mar 30, 2023, Hou Wenlong wrote:
> __kvm_update_cpuid_runtime() may be called during vCPU running and KVM
> PV feature CPUID is updated too. But the cached KVM PV feature bitmap is
> not updated. Actually, KVM PV feature CPUID shouldn't be updated,
> otherwise, KVM PV feature would be broken in guest. Currently, only
> KVM_FEATURE_PV_UNHALT is updated, and it's impossible after disallow
> disable HLT exits. However, KVM PV feature CPUID should be updated only
> in KVM_SET_CPUID{,2} ioctl.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@xxxxxxxxxxxx>
> ---
>  arch/x86/kvm/cpuid.c | 17 ++++++++++++-----
>  1 file changed, 12 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6972e0be60fa..af92d3422c79 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -222,6 +222,17 @@ static struct kvm_cpuid_entry2 *kvm_find_kvm_cpuid_features(struct kvm_vcpu *vcp
>  					     vcpu->arch.cpuid_nent);
>  }
>  
> +static void kvm_update_pv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *entries,
> +				int nent)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
> +	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> +		(best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> +		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> +}
> +
>  void kvm_update_pv_runtime(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpuid_entry2 *best = kvm_find_kvm_cpuid_features(vcpu);
> @@ -280,11 +291,6 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>  		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
>  		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
>  
> -	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
> -	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> -		(best->eax & (1 << KVM_FEATURE_PV_UNHALT)))
> -		best->eax &= ~(1 << KVM_FEATURE_PV_UNHALT);
> -
>  	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_MISC_ENABLE_NO_MWAIT)) {
>  		best = cpuid_entry2_find(entries, nent, 0x1, KVM_CPUID_INDEX_NOT_SIGNIFICANT);
>  		if (best)
> @@ -402,6 +408,7 @@ static int kvm_set_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid_entry2 *e2,
>  	int r;
>  
>  	__kvm_update_cpuid_runtime(vcpu, e2, nent);
> +	kvm_update_pv_cpuid(vcpu, e2, nent);

Hrm, this will silently conflict with the proposed per-vCPU controls[*].  Though
arguably that patch is buggy and "needs" to toggle PV_UNHALT when userspace
messes with HLT passthrough.  But that doesn't really make sense either because
no guest will react kindly to KVM_FEATURE_PV_UNHALT disappearing.

I really wish this code didn't exist, i.e. that KVM let/forced userspace deal
with correctly defining guest CPUID.

Kechen, is it feasible for your userspace to clear PV_UNHALT when it (might) use
the per-vCPU control?  I.e. can KVM do as this series proposes and update guest
CPUID only on KVM_SET_CPUID{2}?  Dropping the behavior for the per-VM control
is probably not an option as I gotta assume that'd break userspace, but I would
really like to avoid carrying that over to the per-vCPU control, which would get
quite messy and probably can't work anyways.

[*] https://lkml.kernel.org/r/20230121020738.2973-6-kechenl%40nvidia.com