This is a note to let you know that I've just added the patch titled
KVM: x86: Use a switch statement and macros in __feature_translate()
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
kvm-x86-use-a-switch-statement-and-macros-in-__feature_translate.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 80c883db87d9ffe2d685e91ba07a087b1c246c78 Mon Sep 17 00:00:00 2001
From: Jim Mattson <jmattson@xxxxxxxxxx>
Date: Mon, 23 Oct 2023 17:16:36 -0700
Subject: KVM: x86: Use a switch statement and macros in __feature_translate()
From: Jim Mattson <jmattson@xxxxxxxxxx>
commit 80c883db87d9ffe2d685e91ba07a087b1c246c78 upstream.
Use a switch statement with macro-generated case statements to handle
translating feature flags in order to reduce the probability of runtime
errors due to copy+paste goofs, to make compile-time errors easier to
debug, and to make the code more readable.
E.g. the compiler won't directly generate an error for duplicate if
statements
if (x86_feature == X86_FEATURE_SGX1)
return KVM_X86_FEATURE_SGX1;
else if (x86_feature == X86_FEATURE_SGX2)
return KVM_X86_FEATURE_SGX1;
and so instead reverse_cpuid_check() will fail due to the untranslated
entry pointing at a Linux-defined leaf, which provides practically no
hint as to what is broken
arch/x86/kvm/reverse_cpuid.h:108:2: error: call to __compiletime_assert_450 declared with 'error' attribute:
BUILD_BUG_ON failed: x86_leaf == CPUID_LNX_4
BUILD_BUG_ON(x86_leaf == CPUID_LNX_4);
^
whereas duplicate case statements very explicitly point at the offending
code:
arch/x86/kvm/reverse_cpuid.h:125:2: error: duplicate case value '361'
KVM_X86_TRANSLATE_FEATURE(SGX2);
^
arch/x86/kvm/reverse_cpuid.h:124:2: error: duplicate case value '360'
KVM_X86_TRANSLATE_FEATURE(SGX1);
^
And without macros, the opposite type of copy+paste goof doesn't generate
any error at compile-time, e.g. this yields no complaints:
case X86_FEATURE_SGX1:
return KVM_X86_FEATURE_SGX1;
case X86_FEATURE_SGX2:
return KVM_X86_FEATURE_SGX1;
Note, __feature_translate() is forcibly inlined and the feature is known
at compile-time, so the code generation between an if-elif sequence and a
switch statement should be identical.
Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20231024001636.890236-2-jmattson@xxxxxxxxxx
[sean: use a macro, rewrite changelog]
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kvm/reverse_cpuid.h | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)
--- a/arch/x86/kvm/reverse_cpuid.h
+++ b/arch/x86/kvm/reverse_cpuid.h
@@ -97,14 +97,16 @@ static __always_inline void reverse_cpui
*/
static __always_inline u32 __feature_translate(int x86_feature)
{
- if (x86_feature == X86_FEATURE_SGX1)
- return KVM_X86_FEATURE_SGX1;
- else if (x86_feature == X86_FEATURE_SGX2)
- return KVM_X86_FEATURE_SGX2;
- else if (x86_feature == X86_FEATURE_RRSBA_CTRL)
- return KVM_X86_FEATURE_RRSBA_CTRL;
+#define KVM_X86_TRANSLATE_FEATURE(f) \
+ case X86_FEATURE_##f: return KVM_X86_FEATURE_##f
- return x86_feature;
+ switch (x86_feature) {
+ KVM_X86_TRANSLATE_FEATURE(SGX1);
+ KVM_X86_TRANSLATE_FEATURE(SGX2);
+ KVM_X86_TRANSLATE_FEATURE(RRSBA_CTRL);
+ default:
+ return x86_feature;
+ }
}
static __always_inline u32 __feature_leaf(int x86_feature)
Patches currently in stable-queue which might be from jmattson@xxxxxxxxxx are
queue-5.15/kvm-x86-advertise-cpuid.-eax-7-ecx-2-edx-to-userspace.patch
queue-5.15/kvm-x86-use-a-switch-statement-and-macros-in-__feature_translate.patch
