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

From: David Laight
Date: Thu Apr 04 2019 - 10:43:42 EST

From: Fenghua Yu
> Sent: 03 April 2019 22:22
> set_cpu_cap() calls locked BTS and clear_cpu_cap() calls locked BTR to
> operate on bitmap defined in x86_capability.
> Locked BTS/BTR accesses a single unsigned long location. In 64-bit mode,
> the location is at:
> base address of x86_capability + (bit offset in x86_capability / 64) * 8
> Since base address of x86_capability may not be aligned to unsigned long,
> the single unsigned long location may cross two cache lines and
> accessing the location by locked BTS/BTR introductions will trigger #AC.

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).


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