[PATCH] x86emul: replace further UB shifts

[PATCH] x86emul: replace further UB shifts

[Jan Beulich]

I have no explanation how I managed to overlook these while putting
together what is now b6a907f8c83d ("x86emul: replace UB shifts").

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -9735,7 +9735,7 @@ x86_emulate(
 
                 rc = ops->read(ea.mem.seg,
                                truncate_ea(ea.mem.off +
-                                           (idx << state->sib_scale)),
+                                           idx * (1 << state->sib_scale)),
                                (void *)mmvalp + i * op_bytes, op_bytes, ctxt);
                 if ( rc != X86EMUL_OKAY )
                 {
@@ -9857,7 +9857,8 @@ x86_emulate(
                 continue;
 
             rc = ops->read(ea.mem.seg,
-                           truncate_ea(ea.mem.off + (idx << state->sib_scale)),
+                           truncate_ea(ea.mem.off +
+                                       idx * (1 << state->sib_scale)),
                            (void *)mmvalp + i * op_bytes, op_bytes, ctxt);
             if ( rc != X86EMUL_OKAY )
             {

Re: [PATCH] x86emul: replace further UB shifts

[Andrew Cooper]

On 04/08/2020 11:13, Jan Beulich wrote:
> I have no explanation how I managed to overlook these while putting
> together what is now b6a907f8c83d ("x86emul: replace UB shifts").
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

I've just checked, and GCC 9.3.0 does convert this pattern back to a
single shift instruction, so at least we're not left with real mul
instructions.

~Andrew