linux-kernel - Re: [PATCH v5] KVM: emulate: Don't inject #GP when emulating RDMPC if CR0.PE=0

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Message-ID: <18aed000-47f7-1fb7-d898-55a94bb1ec74@redhat.com>
Date:   Wed, 20 Oct 2021 12:45:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v5] KVM: emulate: Don't inject #GP when emulating RDMPC if
 CR0.PE=0

On 20/10/21 12:13, 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 a comment why CR0.PE isn't tested since it's impossible for CPL to be >0 if
> CR0.PE=0.
> 
> Reviewed-by: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
> v4 -> v5:
>   * just comments
> v3 -> v4:
>   * add comments instead of pseudocode
> v2 -> v3:
>   * add the missing 'S'
> v1 -> v2:
>   * update patch description
> 
>   arch/x86/kvm/emulate.c | 3 +++
>   1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 9a144ca8e146..c289809beea3 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -4222,6 +4222,9 @@ static int check_rdpmc(struct x86_emulate_ctxt *ctxt)
>   	if (enable_vmware_backdoor && is_vmware_backdoor_pmc(rcx))
>   		return X86EMUL_CONTINUE;
>   
> +	/*
> +	 * It's impossible for CPL to be >0 if CR0.PE=0.
> +	 */
>   	if ((!(cr4 & X86_CR4_PCE) && ctxt->ops->cpl(ctxt)) ||
>   	    ctxt->ops->check_pmc(ctxt, rcx))
>   		return emulate_gp(ctxt, 0);
> 

Queued, thanks (with a slightly more verbose comment).

Paolo