linux-kernel - Re: [PATCH] KVM/VMX: Avoid CR3 VMEXIT during guest real mode when "unrestricted guest" is supported.

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Date:   Thu, 17 Aug 2017 13:08:21 +0800
From:   Lan Tianyu <tianyu.lan@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM/VMX: Avoid CR3 VMEXIT during guest real mode when
 "unrestricted guest" is supported.

On 2017年08月16日 17:25, Paolo Bonzini wrote:
> On 16/08/2017 03:58, Lan Tianyu wrote:
>> These CR3 VMEXITs was introduced for platform without "unrestricted guest"
>> support. This is to set ept identity table to guest CR3 in guest real
>> mode because these platforms don't support ept real mode(CR0.PE and CR0.PG
>> must be set to 1). But these VMEXITs is redundant for platforms with
>> "unrestricted guest" support.
> 
> This is true, but I'm not sure it's a good idea to make things more
> complex...  It is working code and is not a bottleneck anywhere.

Yes, this is just code clear up and will not affect function.
I thought this change was missed when introduced unrestricted guest support.

-- 
Best regards
Tianyu Lan
> 
> Paolo
> 
>> Signed-off-by: Lan Tianyu <tianyu.lan@...el.com>
>> ---
>>  arch/x86/kvm/vmx.c | 22 +++++++++++++---------
>>  1 file changed, 13 insertions(+), 9 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index 9b21b12..46dcf50 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -4221,18 +4221,20 @@ static void ept_update_paging_mode_cr0(unsigned long *hw_cr0,
>>  		vmx_decache_cr3(vcpu);
>>  	if (!(cr0 & X86_CR0_PG)) {
>>  		/* From paging/starting to nonpaging */
>> -		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
>> -			     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) |
>> -			     (CPU_BASED_CR3_LOAD_EXITING |
>> -			      CPU_BASED_CR3_STORE_EXITING));
>> +		if (!enable_unrestricted_guest)
>> +			vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
>> +				     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) |
>> +				     (CPU_BASED_CR3_LOAD_EXITING |
>> +				      CPU_BASED_CR3_STORE_EXITING));
>>  		vcpu->arch.cr0 = cr0;
>>  		vmx_set_cr4(vcpu, kvm_read_cr4(vcpu));
>>  	} else if (!is_paging(vcpu)) {
>>  		/* From nonpaging to paging */
>> -		vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
>> -			     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
>> -			     ~(CPU_BASED_CR3_LOAD_EXITING |
>> -			       CPU_BASED_CR3_STORE_EXITING));
>> +		if (!enable_unrestricted_guest)
>> +			vmcs_write32(CPU_BASED_VM_EXEC_CONTROL,
>> +				     vmcs_read32(CPU_BASED_VM_EXEC_CONTROL) &
>> +				     ~(CPU_BASED_CR3_LOAD_EXITING |
>> +				       CPU_BASED_CR3_STORE_EXITING));
>>  		vcpu->arch.cr0 = cr0;
>>  		vmx_set_cr4(vcpu, kvm_read_cr4(vcpu));
>>  	}
>> @@ -4311,7 +4313,9 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>>  	}
>>  
>>  	vmx_flush_tlb(vcpu);
>> -	vmcs_writel(GUEST_CR3, guest_cr3);
>> +
>> +	if (!enable_unrestricted_guest || !enable_ept)
>> +		vmcs_writel(GUEST_CR3, guest_cr3);
>>  }
>>  
>>  static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>>
>