linux-kernel - Re: [RFC PATCH 2/3] x86/cpufeatures: Combine word 11 and 12 into new scattered features word 11

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Date:   Fri, 14 Jun 2019 15:41:23 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, H Peter Anvin <hpa@...or.com>,
        Christopherson Sean J <sean.j.christopherson@...el.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH 2/3] x86/cpufeatures: Combine word 11 and 12 into new
 scattered features word 11

+ Radim and Paolo. See upthread for context.

On Fri, Jun 14, 2019 at 06:17:02AM -0700, Fenghua Yu wrote:
> > Alternatively - and what I think is the better solution - would be to
> > remove those BUILD_BUG_ONs in x86_feature_cpuid and filter out the
> > Linux-defined leafs dynamically. This way the array won't have holes in
> > it.
> 
> Maybe adding a dummy slot in cpuid_leafs in patch 0002 to avoid the
> compilation errors?

Maybe you didn't read what you're replying to: "This way the array
won't have holes in it". Ontop of yours:

diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index d78a61408243..03d6f3f7b27c 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -47,6 +47,7 @@ static const struct cpuid_reg reverse_cpuid[] = {
 	[CPUID_8000_0001_ECX] = {0x80000001, 0, CPUID_ECX},
 	[CPUID_7_0_EBX]       = {         7, 0, CPUID_EBX},
 	[CPUID_D_1_EAX]       = {       0xd, 1, CPUID_EAX},
+	[CPUID_7_1_EAX]       = {         7, 1, CPUID_EAX},
 	[CPUID_8000_0008_EBX] = {0x80000008, 0, CPUID_EBX},
 	[CPUID_6_EAX]         = {         6, 0, CPUID_EAX},
 	[CPUID_8000_000A_EDX] = {0x8000000a, 0, CPUID_EDX},
@@ -59,8 +60,9 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned x86_feature)
 {
 	unsigned x86_leaf = x86_feature / 32;
 
-	BUILD_BUG_ON(x86_leaf >= ARRAY_SIZE(reverse_cpuid));
-	BUILD_BUG_ON(reverse_cpuid[x86_leaf].function == 0);
+	if (x86_leaf == CPUID_LNX_1 ||
+	    x86_leaf == CPUID_LNX_4)
+		return NULL;
 
 	return reverse_cpuid[x86_leaf];
 }

That's what I mean with filter out dynamically.

-- 
Regards/Gruss,
    Boris.

Good mailing practices for 400: avoid top-posting and trim the reply.