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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 28/09/2020 15:49, Jan Beulich wrote:
> 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.

3 is fine.  5 is a little more problematic, because there are
serialising side effects, but I suppose we really don't care here.

~Andrew