linux-kernel - Re: [PATCH v16 18/51] KVM: x86: Don't emulate instructions affected by CET features

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Message-ID: <aNLHnFa4mD3mA8Gd@google.com>
Date: Tue, 23 Sep 2025 09:15:24 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xiaoyao Li <xiaoyao.li@...el.com>
Cc: Chao Gao <chao.gao@...el.com>, Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Tom Lendacky <thomas.lendacky@....com>, 
	Mathias Krause <minipli@...ecurity.net>, John Allen <john.allen@....com>, 
	Rick Edgecombe <rick.p.edgecombe@...el.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Maxim Levitsky <mlevitsk@...hat.com>, Zhang Yi Z <yi.z.zhang@...ux.intel.com>, Xin Li <xin@...or.com>
Subject: Re: [PATCH v16 18/51] KVM: x86: Don't emulate instructions affected
 by CET features

On Tue, Sep 23, 2025, Xiaoyao Li wrote:
> On 9/23/2025 4:04 AM, Sean Christopherson wrote:
> Two nits besides,
> > Link:https://lore.kernel.org/r/20250919223258.1604852-19-seanjc@google.com
> > Signed-off-by: Sean Christopherson<seanjc@...gle.com>
> > ---
> >   arch/x86/kvm/emulate.c | 117 ++++++++++++++++++++++++++++++++++++-----
> >   1 file changed, 103 insertions(+), 14 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> > index 23929151a5b8..a7683dc18405 100644
> > --- a/arch/x86/kvm/emulate.c
> > +++ b/arch/x86/kvm/emulate.c
> > @@ -178,6 +178,7 @@
> >   #define IncSP       ((u64)1 << 54)  /* SP is incremented before ModRM calc */
> >   #define TwoMemOp    ((u64)1 << 55)  /* Instruction has two memory operand */
> >   #define IsBranch    ((u64)1 << 56)  /* Instruction is considered a branch. */
> > +#define ShadowStack ((u64)1 << 57)  /* Instruction affects Shadow Stacks. */
> >   #define DstXacc     (DstAccLo | SrcAccHi | SrcWrite)
> > @@ -4068,9 +4069,9 @@ 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, em_call_near_abs),
> > +	I(SrcMemFAddr | ImplicitOps | IsBranch | ShadowStack, em_call_far),
> > +	I(SrcMem | NearBranch | IsBranch, em_jmp_abs),
> 
> The change of this line is unexpected, since it only changes the indentation
> of 'em_jmp_abs'
> >   static unsigned imm_size(struct x86_emulate_ctxt *ctxt)
> >   {
> >   	unsigned size;
> > @@ -4943,6 +4998,40 @@ int x86_decode_insn(struct x86_emulate_ctxt *ctxt, void *insn, int insn_len, int
> >   	ctxt->execute = opcode.u.execute;
> > +	/*
> > +	 * Reject emulation if KVM might need to emulate shadow stack updates
> > +	 * and/or indirect branch tracking enforcement, which the emulator
> > +	 * doesn't support.
> > +	 */
> > +	if ((is_ibt_instruction(ctxt) || is_shstk_instruction(ctxt)) &&
> > +	    ctxt->ops->get_cr(ctxt, 4) & X86_CR4_CET) {
> > +		u64 u_cet = 0, s_cet = 0;
> > +
> > +		/*
> > +		 * Check both User and Supervisor on far transfers as inter-
> > +		 * privilege level transfers are impacted by CET at the target
> > +		 * privilege level, and that is not known at this time.  The
> > +		 * the expectation is that the guest will not require emulation
> 
> Dobule 'the'

Squashed fixes for both, thanks!