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

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Date:   Tue, 18 Apr 2017 15:30:15 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>,
        David Hildenbrand <david@...hat.com>
Cc:     kvm@...r.kernel.org, dvyukov@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] KVM: VMX: drop vmm_exclusive module parameter



On 14/03/2017 21:30, Radim Krčmář wrote:
> 2017-03-10 12:47+0100, David Hildenbrand:
>> 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.
> 
> Totally,
> 
> Reviewed-by: Radim Krčmář <rkrcmar@...hat.com>

Radim,

are you putting this in kvm/queue for 4.12?

Thanks,

Paolo

>> 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);
>> -- 
>> 2.9.3
>>