linux-kernel - [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

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Date:   Fri, 12 Jul 2019 15:32:43 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     Arnd Bergmann <arnd@...db.de>, "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        Liran Alon <liran.alon@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning

clang points out that running a 64-bit guest on a 32-bit host
would lead to uninitialized variables:

arch/x86/kvm/hyperv.c:1610:6: error: variable 'ingpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
        if (!longmode) {
            ^~~~~~~~~
arch/x86/kvm/hyperv.c:1632:55: note: uninitialized use occurs here
        trace_kvm_hv_hypercall(code, fast, rep_cnt, rep_idx, ingpa, outgpa);
                                                             ^~~~~
arch/x86/kvm/hyperv.c:1610:2: note: remove the 'if' if its condition is always true
        if (!longmode) {
        ^~~~~~~~~~~~~~~
arch/x86/kvm/hyperv.c:1595:18: note: initialize the variable 'ingpa' to silence this warning
        u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
                        ^
                         = 0
arch/x86/kvm/hyperv.c:1610:6: error: variable 'outgpa' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
arch/x86/kvm/hyperv.c:1610:6: error: variable 'param' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]

Since that combination is not supported anyway, change the condition
to tell the compiler how the code is actually executed.

Signed-off-by: Arnd Bergmann <arnd@...db.de>
---
v2: make the change inside of is_64_bit_mode().
---
 arch/x86/kvm/hyperv.c | 6 ++----
 arch/x86/kvm/x86.h    | 4 ++++
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index a39e38f13029..4c1c423a3d08 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -1594,7 +1594,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 {
 	u64 param, ingpa, outgpa, ret = HV_STATUS_SUCCESS;
 	uint16_t code, rep_idx, rep_cnt;
-	bool fast, longmode, rep;
+	bool fast, rep;
 
 	/*
 	 * hypercall generates UD from non zero cpl and real mode
@@ -1605,9 +1605,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
 		return 1;
 	}
 
-	longmode = is_64_bit_mode(vcpu);
-
-	if (!longmode) {
+	if (!is_64_bit_mode(vcpu)) {
 		param = ((u64)kvm_rdx_read(vcpu) << 32) |
 			(kvm_rax_read(vcpu) & 0xffffffff);
 		ingpa = ((u64)kvm_rbx_read(vcpu) << 32) |
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index e08a12892e8b..b457b3267296 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -90,6 +90,7 @@ static inline int is_long_mode(struct kvm_vcpu *vcpu)
 #endif
 }
 
+#ifdef CONFIG_X86_64
 static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
 {
 	int cs_db, cs_l;
@@ -99,6 +100,9 @@ static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
 	kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
 	return cs_l;
 }
+#else
+#define is_64_bit_mode(vcpu) false
+#endif
 
 static inline bool is_la57_mode(struct kvm_vcpu *vcpu)
 {
-- 
2.20.0