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 13:00:11 +0800
From:   Lan Tianyu <tianyu.lan@...el.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     pbonzini@...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 21:26, Radim Krčmář wrote:
> 2017-08-15 21:58-0400, Lan Tianyu:
>> 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.
>>
>> 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
>> @@ -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);
> 
> This looks wrong -- it would prevent update GUEST_CR3 outside of
> non-root mode with enable_unrestricted_guest.
> 

OK. Do you mean nest mode? I didn't consider that case.
I thought there were three cases here.

1) Shadow page mode(enable_ept=0)

2) ept mode without unrestricted guest mode
   (ept=1, enable_unrestricted_guest = 0)

3) ept mode with unrestricted guest mode
   (ept=1, enable_unrestricted_guest = 1)

>From my understanding, only (1) and (2) need to update guest cr3.
If nest mode is still needed to update guest CR3, we can add
is_guest_mode() in the if condition. Other choice is to just ignore
setting guest cr3 for case3. The condition maybe changed to

if (!(enable_unrestricted_guest && enable_ept))
	vmcs_writel(GUEST_CR3, guest_cr3);





-- 
Best regards
Tianyu Lan