[PATCH] x86/coco: Define cc_vendor without CONFIG_ARCH_HAS_CC_PLATFORM

[Nathan Chancellor]

After commit a9ef277488cf ("x86/kvm: Fix SEV check in
sev_map_percpu_data()"), there is a build error when building
x86_64_defconfig with GCOV using LLVM:

  ld.lld: error: undefined symbol: cc_vendor
  >>> referenced by kvm.c
  >>>               arch/x86/kernel/kvm.o:(kvm_smp_prepare_boot_cpu) in archive vmlinux.a

which corresponds to

  if (cc_vendor != CC_VENDOR_AMD ||
      !cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT))
            return;

Without GCOV, clang is able to eliminate the use of cc_vendor because
cc_platform_has() evaluates to false when CONFIG_ARCH_HAS_CC_PLATFORM is
not set, meaning that if statement will be true no matter what value
cc_vendor has.

With GCOV, the instrumentation keeps the use of cc_vendor around for
code coverage purposes but cc_vendor is only declared, not defined,
without CONFIG_ARCH_HAS_CC_PLATFORM, leading to the build error above.

Provide a macro definition of cc_vendor when CONFIG_ARCH_HAS_CC_PLATFORM
is not set with a value of CC_VENDOR_NONE, so that the first condition
can always be evaluated/eliminated at compile time, avoiding the build
error altogether. This is very similar to the situation prior to
commit da86eb961184 ("x86/coco: Get rid of accessor functions").

Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx>
---
Commit a9ef277488cf ("x86/kvm: Fix SEV check in sev_map_percpu_data()")
exposes this build error but I think it is really a problem with commit
da86eb961184 ("x86/coco: Get rid of accessor functions"), although I am
not positive so I left out the fixes tag. It would be nice if this could
go along with KVM tree that has that change but it is obviously well
within -tip's territory so I will just aim it at both parties and let
them figure it out :)
---
 arch/x86/include/asm/coco.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/coco.h b/arch/x86/include/asm/coco.h
index 6ae2d16a7613..76c310b19b11 100644
--- a/arch/x86/include/asm/coco.h
+++ b/arch/x86/include/asm/coco.h
@@ -10,13 +10,14 @@ enum cc_vendor {
 	CC_VENDOR_INTEL,
 };
 
-extern enum cc_vendor cc_vendor;
-
 #ifdef CONFIG_ARCH_HAS_CC_PLATFORM
+extern enum cc_vendor cc_vendor;
 void cc_set_mask(u64 mask);
 u64 cc_mkenc(u64 val);
 u64 cc_mkdec(u64 val);
 #else
+#define cc_vendor (CC_VENDOR_NONE)
+
 static inline u64 cc_mkenc(u64 val)
 {
 	return val;

---
base-commit: a9ef277488cfc1b7da88235dc11c338a14f34835
change-id: 20240202-provide-cc_vendor-without-arch_has_cc_platform-325a3fae8550

Best regards,
-- 
Nathan Chancellor <nathan@xxxxxxxxxx>