Clang reports the following build error without CONFIG_PV:

hypercall.c:253:10: error: variable 'op' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
    if ( !is_pv_32bit_vcpu(curr) )
         ^~~~~~~~~~~~~~~~~~~~~~~
hypercall.c:282:21: note: uninitialized use occurs here
    return unlikely(op == __HYPERVISOR_iret)
                    ^~
/root/src/xen/xen/include/xen/compiler.h:21:43: note: expanded from macro 'unlikely'
#define unlikely(x)   __builtin_expect(!!(x),0)
                                          ^
hypercall.c:253:5: note: remove the 'if' if its condition is always true
    if ( !is_pv_32bit_vcpu(curr) )
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
hypercall.c:251:21: note: initialize the variable 'op' to silence this warning
    unsigned long op;
                    ^
                     = 0

Rearrange the code in arch_do_multicall_call so that the if guards the
32bit branch and when CONFIG_PV32 is not set there's no conditional at
all.

Fixes: 527922008bc ('x86: slim down hypercall handling when !PV32')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
---
Should the is_pv_32bit_vcpu be wrapped in an unlikely hint?
---
 xen/arch/x86/pv/hypercall.c | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/pv/hypercall.c b/xen/arch/x86/pv/hypercall.c
index e30c59b6286..d573f74aa1e 100644
--- a/xen/arch/x86/pv/hypercall.c
+++ b/xen/arch/x86/pv/hypercall.c
@@ -250,34 +250,34 @@ enum mc_disposition arch_do_multicall_call(struct mc_state *state)
     struct vcpu *curr = current;
     unsigned long op;
 
-    if ( !is_pv_32bit_vcpu(curr) )
+#ifdef CONFIG_PV32
+    if ( is_pv_32bit_vcpu(curr) )
     {
-        struct multicall_entry *call = &state->call;
+        struct compat_multicall_entry *call = &state->compat_call;
 
         op = call->op;
         if ( (op < ARRAY_SIZE(pv_hypercall_table)) &&
-             pv_hypercall_table[op].native )
-            call->result = pv_hypercall_table[op].native(
+             pv_hypercall_table[op].compat )
+            call->result = pv_hypercall_table[op].compat(
                 call->args[0], call->args[1], call->args[2],
                 call->args[3], call->args[4], call->args[5]);
         else
             call->result = -ENOSYS;
     }
-#ifdef CONFIG_PV32
     else
+#endif
     {
-        struct compat_multicall_entry *call = &state->compat_call;
+        struct multicall_entry *call = &state->call;
 
         op = call->op;
         if ( (op < ARRAY_SIZE(pv_hypercall_table)) &&
-             pv_hypercall_table[op].compat )
-            call->result = pv_hypercall_table[op].compat(
+             pv_hypercall_table[op].native )
+            call->result = pv_hypercall_table[op].native(
                 call->args[0], call->args[1], call->args[2],
                 call->args[3], call->args[4], call->args[5]);
         else
             call->result = -ENOSYS;
     }
-#endif
 
     return unlikely(op == __HYPERVISOR_iret)
            ? mc_exit
-- 
2.30.1

On Fri, Apr 23, 2021 at 11:43:43AM +0200, Roger Pau Monne wrote:
> Clang reports the following build error without CONFIG_PV:

s/CONFIG_PV/CONFIG_PV32/ on the line above, sorry.

Thanks, Roger.

On 23.04.2021 11:43, Roger Pau Monne wrote:
> Clang reports the following build error without CONFIG_PV:
> 
> hypercall.c:253:10: error: variable 'op' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
>     if ( !is_pv_32bit_vcpu(curr) )
>          ^~~~~~~~~~~~~~~~~~~~~~~
> hypercall.c:282:21: note: uninitialized use occurs here
>     return unlikely(op == __HYPERVISOR_iret)
>                     ^~
> /root/src/xen/xen/include/xen/compiler.h:21:43: note: expanded from macro 'unlikely'
> #define unlikely(x)   __builtin_expect(!!(x),0)
>                                           ^
> hypercall.c:253:5: note: remove the 'if' if its condition is always true
>     if ( !is_pv_32bit_vcpu(curr) )
>     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> hypercall.c:251:21: note: initialize the variable 'op' to silence this warning
>     unsigned long op;
>                     ^
>                      = 0
> 
> Rearrange the code in arch_do_multicall_call so that the if guards the
> 32bit branch and when CONFIG_PV32 is not set there's no conditional at
> all.
> 
> Fixes: 527922008bc ('x86: slim down hypercall handling when !PV32')
> Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>

I find it odd for the compiler to warn like this, but well ...
Acked-by: Jan Beulich <jbeulich@suse.com>

> Should the is_pv_32bit_vcpu be wrapped in an unlikely hint?

Not sure. Andrew did some similar rearrangement elsewhere for
other reasons, without adding unlikely(). Personally I think
we'd better add them, but then preferably add consistently as
possible.

Jan