linux-kernel - Re: [PATCH RFC] KVM: VMX: drop vmm

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Date:   Fri, 21 Apr 2017 11:42:55 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org
Cc:     rkrcmar@...hat.com, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] KVM: VMX: drop vmm_exclusive module parameter



On 10/03/2017 12:47, David Hildenbrand wrote:
> vmm_exclusive=0 leads to KVM setting X86_CR4_VMXE always and calling
> VMXON only when the vcpu is loaded. X86_CR4_VMXE is used as an
> indication in cpu_emergency_vmxoff() (called on kdump) if VMXOFF has to be
> called. This is obviously not the case if both are used independtly.
> Calling VMXOFF without a previous VMXON will result in an exception.
> 
> In addition, X86_CR4_VMXE is used as a mean to test if VMX is already in
> use by another VMM in hardware_enable(). So there can't really be
> co-existance. If the other VMM is prepared for co-existance and does a
> similar check, only one VMM can exist. If the other VMM is not prepared
> and blindly sets/clears X86_CR4_VMXE, we will get inconsistencies with
> X86_CR4_VMXE.
> 
> As we also had bug reports related to clearing of vmcs with vmm_exclusive=0
> this seems to be pretty much untested. So let's better drop it.
> 
> While at it, directly move setting/clearing X86_CR4_VMXE into
> kvm_cpu_vmxon/off.
> 
> Signed-off-by: David Hildenbrand <david@...hat.com>
> ---
>  arch/x86/kvm/vmx.c | 38 +++++++-------------------------------
>  1 file changed, 7 insertions(+), 31 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index 283aa86..bbbfe12 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -84,9 +84,6 @@ module_param_named(eptad, enable_ept_ad_bits, bool, S_IRUGO);
>  static bool __read_mostly emulate_invalid_guest_state = true;
>  module_param(emulate_invalid_guest_state, bool, S_IRUGO);
>  
> -static bool __read_mostly vmm_exclusive = 1;
> -module_param(vmm_exclusive, bool, S_IRUGO);
> -
>  static bool __read_mostly fasteoi = 1;
>  module_param(fasteoi, bool, S_IRUGO);
>  
> @@ -914,8 +911,6 @@ static void nested_release_page_clean(struct page *page)
>  
>  static unsigned long nested_ept_get_cr3(struct kvm_vcpu *vcpu);
>  static u64 construct_eptp(unsigned long root_hpa);
> -static void kvm_cpu_vmxon(u64 addr);
> -static void kvm_cpu_vmxoff(void);
>  static bool vmx_xsaves_supported(void);
>  static int vmx_set_tss_addr(struct kvm *kvm, unsigned int addr);
>  static void vmx_set_segment(struct kvm_vcpu *vcpu,
> @@ -2235,15 +2230,10 @@ static void decache_tsc_multiplier(struct vcpu_vmx *vmx)
>  static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> -	u64 phys_addr = __pa(per_cpu(vmxarea, cpu));
>  	bool already_loaded = vmx->loaded_vmcs->cpu == cpu;
>  
> -	if (!vmm_exclusive)
> -		kvm_cpu_vmxon(phys_addr);
> -	else if (!already_loaded)
> -		loaded_vmcs_clear(vmx->loaded_vmcs);
> -
>  	if (!already_loaded) {
> +		loaded_vmcs_clear(vmx->loaded_vmcs);
>  		local_irq_disable();
>  		crash_disable_local_vmclear(cpu);
>  
> @@ -2321,11 +2311,6 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
>  	vmx_vcpu_pi_put(vcpu);
>  
>  	__vmx_load_host_state(to_vmx(vcpu));
> -	if (!vmm_exclusive) {
> -		__loaded_vmcs_clear(to_vmx(vcpu)->loaded_vmcs);
> -		vcpu->cpu = -1;
> -		kvm_cpu_vmxoff();
> -	}
>  }
>  
>  static void vmx_decache_cr0_guest_bits(struct kvm_vcpu *vcpu);
> @@ -3416,6 +3401,7 @@ static __init int vmx_disabled_by_bios(void)
>  
>  static void kvm_cpu_vmxon(u64 addr)
>  {
> +	cr4_set_bits(X86_CR4_VMXE);
>  	intel_pt_handle_vmx(1);
>  
>  	asm volatile (ASM_VMX_VMXON_RAX
> @@ -3458,12 +3444,8 @@ static int hardware_enable(void)
>  		/* enable and lock */
>  		wrmsrl(MSR_IA32_FEATURE_CONTROL, old | test_bits);
>  	}
> -	cr4_set_bits(X86_CR4_VMXE);
> -
> -	if (vmm_exclusive) {
> -		kvm_cpu_vmxon(phys_addr);
> -		ept_sync_global();
> -	}
> +	kvm_cpu_vmxon(phys_addr);
> +	ept_sync_global();
>  
>  	native_store_gdt(this_cpu_ptr(&host_gdt));
>  
> @@ -3489,15 +3471,13 @@ static void kvm_cpu_vmxoff(void)
>  	asm volatile (__ex(ASM_VMX_VMXOFF) : : : "cc");
>  
>  	intel_pt_handle_vmx(0);
> +	cr4_clear_bits(X86_CR4_VMXE);
>  }
>  
>  static void hardware_disable(void)
>  {
> -	if (vmm_exclusive) {
> -		vmclear_local_loaded_vmcss();
> -		kvm_cpu_vmxoff();
> -	}
> -	cr4_clear_bits(X86_CR4_VMXE);
> +	vmclear_local_loaded_vmcss();
> +	kvm_cpu_vmxoff();
>  }
>  
>  static __init int adjust_vmx_controls(u32 ctl_min, u32 ctl_opt,
> @@ -9228,11 +9208,7 @@ static struct kvm_vcpu *vmx_create_vcpu(struct kvm *kvm, unsigned int id)
>  	vmx->loaded_vmcs->shadow_vmcs = NULL;
>  	if (!vmx->loaded_vmcs->vmcs)
>  		goto free_msrs;
> -	if (!vmm_exclusive)
> -		kvm_cpu_vmxon(__pa(per_cpu(vmxarea, raw_smp_processor_id())));
>  	loaded_vmcs_init(vmx->loaded_vmcs);
> -	if (!vmm_exclusive)
> -		kvm_cpu_vmxoff();
>  
>  	cpu = get_cpu();
>  	vmx_vcpu_load(&vmx->vcpu, cpu);
> 

Applied, thanks.

Paolo