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

[David Laight]

From: David Laight
> Sent: 04 April 2019 15:45
> 
> 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).

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.

	David

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