linux-kernel - Re: [PATCH] kvm: x86: svm: remove SVM_EXIT_READ

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Date:	Thu, 12 Mar 2015 17:17:16 -0400
From:	Bandan Das <bsd@...hat.com>
To:	Joel Schopp <joel.schopp@....com>
Cc:	Gleb Natapov <gleb@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
	Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
	<linux-kernel@...r.kernel.org>,
	David Kaplan <david.kaplan@....com>, <rkrcmar@...hat.com>
Subject: Re: [PATCH] kvm: x86: svm: remove SVM_EXIT_READ_CR* intercepts


Joel Schopp <joel.schopp@....com> writes:

> There isn't really a valid reason for kvm to intercept cr* reads
> on svm hardware.  The current kvm code just ends up returning
> the register
>
> Signed-off-by: Joel Schopp <joel.schopp@....com>
> ---
>  arch/x86/kvm/svm.c |   41 ++++-------------------------------------
>  1 file changed, 4 insertions(+), 37 deletions(-)
>
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index cc618c8..c3d10e6 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -1090,9 +1090,6 @@ static void init_vmcb(struct vcpu_svm *svm)
>  	svm->vcpu.fpu_active = 1;
>  	svm->vcpu.arch.hflags = 0;
>  
> -	set_cr_intercept(svm, INTERCEPT_CR0_READ);
> -	set_cr_intercept(svm, INTERCEPT_CR3_READ);
> -	set_cr_intercept(svm, INTERCEPT_CR4_READ);
>  	set_cr_intercept(svm, INTERCEPT_CR0_WRITE);
>  	set_cr_intercept(svm, INTERCEPT_CR3_WRITE);
>  	set_cr_intercept(svm, INTERCEPT_CR4_WRITE);
> @@ -1174,7 +1171,6 @@ static void init_vmcb(struct vcpu_svm *svm)
>  		control->nested_ctl = 1;
>  		clr_intercept(svm, INTERCEPT_INVLPG);
>  		clr_exception_intercept(svm, PF_VECTOR);
> -		clr_cr_intercept(svm, INTERCEPT_CR3_READ);
>  		clr_cr_intercept(svm, INTERCEPT_CR3_WRITE);
>  		save->g_pat = 0x0007040600070406ULL;
>  		save->cr3 = 0;
> @@ -2968,29 +2964,10 @@ static int cr_interception(struct vcpu_svm *svm)
>  			kvm_queue_exception(&svm->vcpu, UD_VECTOR);
>  			return 1;
>  		}
> -	} else { /* mov from cr */
> -		switch (cr) {
> -		case 0:
> -			val = kvm_read_cr0(&svm->vcpu);
> -			break;
> -		case 2:
> -			val = svm->vcpu.arch.cr2;
> -			break;
> -		case 3:
> -			val = kvm_read_cr3(&svm->vcpu);
> -			break;
> -		case 4:
> -			val = kvm_read_cr4(&svm->vcpu);
> -			break;
> -		case 8:
> -			val = kvm_get_cr8(&svm->vcpu);
> -			break;
> -		default:
> -			WARN(1, "unhandled read from CR%d", cr);
> -			kvm_queue_exception(&svm->vcpu, UD_VECTOR);
> -			return 1;
> -		}
> -		kvm_register_write(&svm->vcpu, reg, val);
> +	} else { /* mov from cr, should never trap in svm */
> +		WARN(1, "unhandled read from CR%d", cr);
> +		kvm_queue_exception(&svm->vcpu, UD_VECTOR);
> +		return 1;

Can we end up here if a nested hypervisor sets cr read interception ?

Bandan

>  	}
>  	kvm_complete_insn_gp(&svm->vcpu, err);
>  
> @@ -3321,10 +3298,6 @@ static int mwait_interception(struct vcpu_svm *svm)
>  }
>  
>  static int (*const svm_exit_handlers[])(struct vcpu_svm *svm) = {
> -	[SVM_EXIT_READ_CR0]			= cr_interception,
> -	[SVM_EXIT_READ_CR3]			= cr_interception,
> -	[SVM_EXIT_READ_CR4]			= cr_interception,
> -	[SVM_EXIT_READ_CR8]			= cr_interception,
>  	[SVM_EXIT_CR0_SEL_WRITE]		= emulate_on_interception,
>  	[SVM_EXIT_WRITE_CR0]			= cr_interception,
>  	[SVM_EXIT_WRITE_CR3]			= cr_interception,
> @@ -4151,11 +4124,9 @@ static const struct __x86_intercept {
>  	u32 exit_code;
>  	enum x86_intercept_stage stage;
>  } x86_intercept_map[] = {
> -	[x86_intercept_cr_read]		= POST_EX(SVM_EXIT_READ_CR0),
>  	[x86_intercept_cr_write]	= POST_EX(SVM_EXIT_WRITE_CR0),
>  	[x86_intercept_clts]		= POST_EX(SVM_EXIT_WRITE_CR0),
>  	[x86_intercept_lmsw]		= POST_EX(SVM_EXIT_WRITE_CR0),
> -	[x86_intercept_smsw]		= POST_EX(SVM_EXIT_READ_CR0),
>  	[x86_intercept_dr_read]		= POST_EX(SVM_EXIT_READ_DR0),
>  	[x86_intercept_dr_write]	= POST_EX(SVM_EXIT_WRITE_DR0),
>  	[x86_intercept_sldt]		= POST_EX(SVM_EXIT_LDTR_READ),
> @@ -4221,10 +4192,6 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu,
>  		goto out;
>  
>  	switch (icpt_info.exit_code) {
> -	case SVM_EXIT_READ_CR0:
> -		if (info->intercept == x86_intercept_cr_read)
> -			icpt_info.exit_code += info->modrm_reg;
> -		break;
>  	case SVM_EXIT_WRITE_CR0: {
>  		unsigned long cr0, val;
>  		u64 intercept;
>
> --
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