Re: [PATCH 1/4] x86/altcall: Check and optimise altcall targets

[Andrew Cooper <amc96@srcf.net>]

On 01/12/2021 08:10, Jan Beulich wrote:
> On 26.11.2021 22:22, Andrew Cooper wrote:
>> @@ -279,6 +280,27 @@ static void init_or_livepatch _apply_alternatives(struct alt_instr *start,
>>  
>>                  if ( dest )
>>                  {
>> +                    /*
>> +                     * When building for CET-IBT, all function pointer targets
>> +                     * should have an endbr64 instruction.
>> +                     *
>> +                     * If this is not the case, leave a warning because
>> +                     * something is wrong with the build.
>> +                     *
>> +                     * Otherwise, skip the endbr64 instruction.  This is a
>> +                     * marginal perf improvement which saves on instruction
>> +                     * decode bandwidth.
>> +                     */
>> +                    if ( IS_ENABLED(CONFIG_HAS_CC_CET_IBT) )
>> +                    {
>> +                        if ( is_endbr64(dest) )
> I would have given my R-b, but I don't see where is_endbr64() is coming
> from, and you don't list any prereqs here or in the cover letter. I'm
> afraid I don't fancy going hunt for it in the many other pending patches.
> Hence only on the assumption that the helper has got introduced before:
> Reviewed-by: Jan Beulich <jbeulich@suse.com>

Oh sorry - this series is based on the CET-IBT series, which adds
CONFIG_HAS_CC_CET_IBT and is_endbr64().

~Andrew