Re: [PATCH 1/5] x86: introduce read_sregs() to allow storing to memory directly

[Jan Beulich <jbeulich@xxxxxxxx>]

On 28.09.2020 14:47, Andrew Cooper wrote:
> On 28/09/2020 13:05, Jan Beulich wrote:
>> --- a/xen/include/asm-x86/regs.h
>> +++ b/xen/include/asm-x86/regs.h
>> @@ -15,4 +15,18 @@
>>      (diff == 0);                                                            
>>   \
>>  })
>>  
>> +#define read_sreg(name) ({                                    \
>> +    unsigned int __sel;                                       \
>> +    asm volatile ( "mov %%" STR(name) ",%0" : "=r" (__sel) ); \
>> +    __sel;                                                    \
>> +})
>> +
>> +static inline void read_sregs(struct cpu_user_regs *regs)
>> +{
>> +    asm volatile ( "mov %%ds, %0" : "=m" (regs->ds) );
>> +    asm volatile ( "mov %%es, %0" : "=m" (regs->es) );
>> +    asm volatile ( "mov %%fs, %0" : "=m" (regs->fs) );
>> +    asm volatile ( "mov %%gs, %0" : "=m" (regs->gs) );
> 
> It occurs to me that reads don't need to be volatile.  There are no side
> effects.

I'll do the same for what patches 3 and 5 alter anyway, assuming
this won't invalidate your R-b there.

Jan