linux-kernel - Re: [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1

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Date:   Fri, 8 Oct 2021 15:20:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH 1/3] KVM: emulate: #GP when emulating rdpmc if CR0.PE is 1

The shortlog makes it sound like "inject a #GP if CR0.PE=1", i.e. unconditionally
inject #GP for RDMPC in protected mode.  Maybe "Don't inject #GP when emulating
RDMPC if CR0.PE=0"?

On Fri, Oct 08, 2021, Wanpeng Li wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> SDM mentioned that, RDPMC: 
> 
>   IF (((CR4.PCE = 1) or (CPL = 0) or (CR0.PE = 0)) and (ECX indicates a supported counter)) 
>       THEN
>           EAX := counter[31:0];
>           EDX := ZeroExtend(counter[MSCB:32]);
>       ELSE (* ECX is not valid or CR4.PCE is 0 and CPL is 1, 2, or 3 and CR0.PE is 1 *)
>           #GP(0); 
>   FI;
> 
> Let's add the CR0.PE is 1 checking to rdpmc emulate.
> 
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/emulate.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 9a144ca8e146..ab7ec569e8c9 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4213,6 +4213,7 @@ static int check_rdtsc(struct x86_emulate_ctxt *ctxt)
>  static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
>  {
>  	u64 cr4 = ctxt->ops->get_cr(ctxt, 4);
> +	u64 cr0 = ctxt->ops->get_cr(ctxt, 0);
>  	u64 rcx = reg_read(ctxt, VCPU_REGS_RCX);
>  
>  	/*
> @@ -4222,7 +4223,7 @@ static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
>  	if (enable_vmware_backdoor && is_vmware_backdoor_pmc(rcx))
>  		return X86EMUL_CONTINUE;
>  
> -	if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt)) ||
> +	if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt) && (cr0 & X86_CR0_PE)) ||

I don't think it's possible for CPL to be >0 if CR0.PE=0, e.g. we could probably
WARN in the #GP path.  Realistically it doesn't add value though, so maybe just
add a blurb in the changelog saying this isn't strictly necessary?

>  	    ctxt->ops->check_pmc(ctxt, rcx))
>  		return emulate_gp(ctxt, 0);
>  
> -- 
> 2.25.1
>