linux-kernel - Re: [PATCH v14 15/22] KVM: x86: Don't emulate instructions guarded by CET

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Message-ID: <aMQxaf6SwMz-RJ0I@google.com>
Date: Fri, 12 Sep 2025 07:42:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, acme@...hat.com, 
	bp@...en8.de, dave.hansen@...ux.intel.com, hpa@...or.com, john.allen@....com, 
	mingo@...nel.org, mingo@...hat.com, minipli@...ecurity.net, 
	mlevitsk@...hat.com, namhyung@...nel.org, pbonzini@...hat.com, 
	prsampat@....com, rick.p.edgecombe@...el.com, shuah@...nel.org, 
	tglx@...utronix.de, weijiang.yang@...el.com, x86@...nel.org, xin@...or.com, 
	xiaoyao.li@...el.com
Subject: Re: [PATCH v14 15/22] KVM: x86: Don't emulate instructions guarded by CET

On Tue, Sep 09, 2025, Chao Gao wrote:
> @@ -4068,9 +4070,11 @@ static const struct opcode group4[] = {
>  static const struct opcode group5[] = {
>  	F(DstMem | SrcNone | Lock,		em_inc),
>  	F(DstMem | SrcNone | Lock,		em_dec),
> -	I(SrcMem | NearBranch | IsBranch,       em_call_near_abs),
> -	I(SrcMemFAddr | ImplicitOps | IsBranch, em_call_far),
> -	I(SrcMem | NearBranch | IsBranch,       em_jmp_abs),
> +	I(SrcMem | NearBranch | IsBranch | ShadowStack | IndirBrnTrk,
> +	em_call_near_abs),

Argh, these wraps are killing me.  I spent a good 20 seconds staring at the code
trying to figure out which instructions are affected.  There's definitely a bit
of -ENOCOFFEE going on, but there's also zero reason to wrap.

> +	I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack | IndirBrnTrk,
> +	em_call_far),
> +	I(SrcMem | NearBranch | IsBranch | IndirBrnTrk, em_jmp_abs),
>  	I(SrcMemFAddr | ImplicitOps | IsBranch, em_jmp_far),
>  	I(SrcMem | Stack | TwoMemOp,		em_push), D(Undefined),
>  };