Re: [Xen-devel] [PATCH v2 1/7] x86: Add support for STAC/CLAC instructions

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 23.04.14 at 14:11, <feng.wu@xxxxxxxxx> wrote:
>> > +#define ASM_AC(op)                                       \
>> > +        pushq %rax;                                      \
>> > +        leaq boot_cpu_data(%rip), %rax;                  \
>> > +        btl $X86_FEATURE_SMAP-7*32, CPUINFO86_leaf7_features(%rax); \
>> > +        jnc 881f;                                        \
>> > +        op;                                              \
>> > +881:    popq %rax
>> 
>> While it is unlikely to change going forwards, $(X86_FEATURE_SMAP & 31)
>> looks more obviously correct.
> 
> Okay.
>> 
>> Also, given that boot_cpu_data(%rip) is an assembler know constant
>> value, and CPUINFO86_leaf7_features is a constant offset from that, is
>> there any way of persuading the assembler to conjoin the two and forgo
>> the push, lea and pop.  (I have had a go, but lack sufficient caffeine
>> this early in the day)
>> 
>> ~Andrew
> 
> I thought about this idea before posting this version, but I didn't get a 
> good
> solution. I will continue to find a better way to handle this.

btl $X86_FEATURE_SMAP & 31, ((X86_FEATURE_SMAP >> 3) & 
~3)+CPUINFO86_features+boot_cpu_data(%rip)

(at once replacing CPUINFO86_ext_features with CPUINFO86_features,
perhaps also changing the [inconsistent] name prefix; likely a suitable
macro could be created usable both here and wherever
CPUINFO86_ext_features is being used currently)

Jan


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