linux-kernel - Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set

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Message-ID: <aee0fe86-6a5a-680a-4147-3b68fc3718c9@redhat.com>
Date:   Wed, 9 Oct 2019 22:59:10 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Reto Buerki <reet@...elabs.ch>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for
 unrestricted guest

On 09/10/19 18:38, Sean Christopherson wrote:
> On Wed, Oct 09, 2019 at 12:40:53PM +0200, Paolo Bonzini wrote:
>> On 27/09/19 23:45, Sean Christopherson wrote:
>>> Rework vmx_set_rflags() to avoid the extra code need to handle emulation
>>> of real mode and invalid state when unrestricted guest is disabled.  The
>>> primary reason for doing so is to avoid the call to vmx_get_rflags(),
>>> which will incur a VMREAD when RFLAGS is not already available.  When
>>> running nested VMs, the majority of calls to vmx_set_rflags() will occur
>>> without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS
>>> during transitions between vmcs01 and vmcs02.
>>>
>>> Note, vmx_get_rflags() guarantees RFLAGS is marked available.
>>
>> Slightly nicer this way:
>>
>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>> index 8de9853d7ab6..62ab19d65efd 100644
>> --- a/arch/x86/kvm/vmx/vmx.c
>> +++ b/arch/x86/kvm/vmx/vmx.c
>> @@ -1431,9 +1431,17 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>>  void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>>  {
>>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
>> -	unsigned long old_rflags = vmx_get_rflags(vcpu);
>> +	unsigned long old_rflags;
>> +
>> +	if (enable_unrestricted_guest) {
>> +		__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>> +		vmx->rflags = rflags;
>> +		vmcs_writel(GUEST_RFLAGS, rflags);
>> +		return;
>> +	}
>> +
>> +	old_rflags = vmx_get_rflags(vcpu);
>>  
>> -	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
>>  	vmx->rflags = rflags;
>>  	if (vmx->rmode.vm86_active) {
>>  		vmx->rmode.save_rflags = rflags;
> 
> Works for me.  Do you want me to spin a v3 to incorporate this and remove
> the open coding of the RIP/RSP accessors?  Or are you planning on squashing
> the changes as you apply?

If it's okay for you I can squash it.

Paolo