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

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Date:   Wed, 16 Aug 2017 15:26:29 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.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.

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.