Re: [PATCH v6 04/20] x86/split_lock: Align x86_capability to unsigned long to avoid split locked access

[Paolo Bonzini]

On 04/04/19 18:52, Thomas Gleixner wrote:
> On Thu, 4 Apr 2019, David Laight wrote:
>> From: David Laight Sent: 04 April 2019 15:45
>>> From: Fenghua Yu Sent: 03 April 2019 22:22
>>> That is not true.
>>> The BTS/BTR instructions access the memory word that contains the
>>> expected bit.
>>> The 'operand size' only affects the size of the register use for the
>>> bit offset.
>>> If the 'operand size' is 16 bits wide (+/- 32k bit offset) the cpu might
>>> do an aligned 16bit memory access, otherwise (32 or 64bit bit offset) it
>>> might do an aligned 32 bit access.
>>> It should never do an 64bit access and never a misaligned one (even if
>>> the base address is misaligned).
>>
>> Hmmm... I may have misread things slightly.
>> The accessed address is 'Effective Address + (4 â (BitOffset DIV 32))'.
>> However nothing suggests that it ever does 64bit accesses.
>>
>> If it does do 64bit accesses when the operand size is 64 bits then the
>> asm stubs ought to be changed to only specify 32bit operand size.
> 
> bitops operate on unsigned long arrays, so the RMW on the affected array
> member has to be atomic vs. other RMW operations on the same array
> member. If we make the bitops 32bit wide on x86/64 we break that.
> 
> So selecting 64bit access (REX.W prefix) is correct and has to stay.

Aren't bitops always atomic with respect to the whole cache line(s)?  We
regularly rely on cmpxchgl being atomic with respect to movb.

Paolo