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SubjectRe: [PATCH 1/2] x86: kvm: avoid -Wsometimes-uninitized warning
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On Fri, Jul 12, 2019 at 11:12:29AM +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.

Hmm, the compiler *is* told all it needs:


arch/x86/kvm/x86.h:
...
static inline int is_long_mode(struct kvm_vcpu *vcpu)
{
#ifdef CONFIG_X86_64
return vcpu->arch.efer & EFER_LMA;
#else
return 0;
#endif
}

static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
{
int cs_db, cs_l;

if (!is_long_mode(vcpu))
return false;
kvm_x86_ops->get_cs_db_l_bits(vcpu, &cs_db, &cs_l);
return cs_l;
}
...

so in !CONFIG_X86_64 case is_64_bit_mode() unconditionally returns
false, and the branch setting the values of the variables is always
taken.

> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> arch/x86/kvm/hyperv.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> index a39e38f13029..950436c502ba 100644
> --- a/arch/x86/kvm/hyperv.c
> +++ b/arch/x86/kvm/hyperv.c
> @@ -1607,7 +1607,7 @@ int kvm_hv_hypercall(struct kvm_vcpu *vcpu)
>
> longmode = is_64_bit_mode(vcpu);
>
> - if (!longmode) {
> + if (!IS_ENABLED(CONFIG_X86_64) || !longmode) {
> param = ((u64)kvm_rdx_read(vcpu) << 32) |
> (kvm_rax_read(vcpu) & 0xffffffff);
> ingpa = ((u64)kvm_rbx_read(vcpu) << 32) |

So this is rather a workaround for the compiler giving false positive.
I suggest to at least rephrase the log message to inidcate this.

Roman.

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