linux-kernel - Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor and Centaur classes

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Message-ID: <5567edf6-a04c-5810-8ed5-78a0db14b202@redhat.com>
Date:   Fri, 6 Mar 2020 10:03:37 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Jim Mattson <jmattson@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, Pu Wen <puwen@...on.cn>
Subject: Re: [PATCH v2 4/7] KVM: x86: Fix CPUID range checks for Hypervisor
 and Centaur classes

On 05/03/20 22:51, Sean Christopherson wrote:
>> Ah. So cross-vendor CPUID specifications are not supported?
> Cross-vendor CPUID is sort of allowed?  E.g. this plays nice with creating
> a Centaur CPU on an Intel platform.  My interpretation of GET_SUPPORTED...
> is that KVM won't prevent enumerating what you want in CPUID, but it only
> promises to correctly support select leafs.

But in practice does this change anything?  If the vendor is not Centaur 
it's unlikely that there is a 0xc0000000 leaf.  The 0x80000000 bound is
certainly not going to be at 0xc0000000 or beyond, and likewise to 0xc0000000
bound is not going to be at 0xd0000000 or beyond.  So I'm not sure if
anything is lost from this simplification:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index ed5e0bda672c..f43a8875c126 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -963,8 +963,7 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
 
 	if (function >= 0x40000000 && function <= 0x4fffffff)
 		class = kvm_find_cpuid_entry(vcpu, function & 0xffffff00, 0);
-	else if (function >= 0xc0000000 && function <= 0xcfffffff &&
-		 is_guest_vendor_centaur(basic->ebx, basic->ecx, basic->edx))
+	else if (function >= 0xc0000000)
 		class = kvm_find_cpuid_entry(vcpu, 0xc0000000, 0);
 	else
 		class = kvm_find_cpuid_entry(vcpu, function & 0x80000000, 0);
diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 12ddfa493bae..3cb50eda606d 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -424,13 +424,6 @@ static inline bool is_guest_vendor_hygon(u32 ebx, u32 ecx, u32 edx)
 	       edx == X86EMUL_CPUID_VENDOR_HygonGenuine_edx;
 }
 
-static inline bool is_guest_vendor_centaur(u32 ebx, u32 ecx, u32 edx)
-{
-	return ebx == X86EMUL_CPUID_VENDOR_CentaurHauls_ebx &&
-	       ecx == X86EMUL_CPUID_VENDOR_CentaurHauls_ecx &&
-	       edx == X86EMUL_CPUID_VENDOR_CentaurHauls_edx;
-}
-
 enum x86_intercept_stage {
 	X86_ICTP_NONE = 0,   /* Allow zero-init to not match anything */
 	X86_ICPT_PRE_EXCEPT,