Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning
On Fri, Jul 12, 2019 at 3:54 PM Roman Kagan wrote:
>
> On Fri, Jul 12, 2019 at 03:32:43PM +0200, Arnd Bergmann wrote:
> > 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
> > ---
> > 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(-)
>
> Reviewed-by: Roman Kagan
>
> However I still think the log message could state it more explicitly
> that it was the compiler's fault, and the patch is a workaround for it.
>
> Otherwise later on someone may decide to restore the similarity of
> is_64_bit_mode() to other inlines in this file, and will be extremely
> unlikely to test clang 32-bit build...
Fair enough. I've reworded the changelog, as well as the patch to
document this now, in a way that should make it harder to introduce
the warning again by accident. Unfortunately, that #ifdef check
cannot be turned into an if(IS_ENABLED()) because kvm_r8_read()
is not defined on i386.
Note that the 0-day bot now tests with clang as well, so you would
definitely hear about a warning appearing.
v3 coming.
Arnd
Re: [PATCH] [v2] x86: kvm: avoid -Wsometimes-uninitized warning
On Fri, Jul 12, 2019 at 03:32:43PM +0200, Arnd Bergmann wrote:
> 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
> ---
> 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(-)
Reviewed-by: Roman Kagan
However I still think the log message could state it more explicitly
that it was the compiler's fault, and the patch is a workaround for it.
Otherwise later on someone may decide to restore the similarity of
is_64_bit_mode() to other inlines in this file, and will be extremely
unlikely to test clang 32-bit build...
Roman.
[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
---
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) & 0x);
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
