Re: [Qemu-devel] [PATCH v3 27/34] target-arm: emulate LL/SC using cmpxchg helpers

[Pranith Kumar]

On Wed, Sep 14, 2016 at 12:38 PM, Richard Henderson <address@hidden> wrote:
> On 09/14/2016 09:03 AM, Alex Bennée wrote:
>>> > -/* Load/Store exclusive instructions are implemented by remembering
>>> > -   the value/address loaded, and seeing if these are the same
>>> > -   when the store is performed. This should be sufficient to implement
>>> > -   the architecturally mandated semantics, and avoids having to monitor
>>> > -   regular stores.
>>> > -
>>> > -   In system emulation mode only one CPU will be running at once, so
>>> > -   this sequence is effectively atomic.  In user emulation mode we
>>> > -   throw an exception and handle the atomic operation elsewhere.  */
>> At least half of this comment is still relevant although it could be
>> tweaked to mention that we use an atomic cmpxchg for the store that will
>> fail if exlusive_val doesn't match the current state.
>>
>
> Added back
>
> /* Load/Store exclusive instructions are implemented by remembering
>    the value/address loaded, and seeing if these are the same
>    when the store is performed.  This should be sufficient to implement
>    the architecturally mandated semantics, and avoids having to monitor
>    regular stores.  The compare vs the remembered value is done during
>    the cmpxchg operation, but we must compare the addresses manually.  */
>

FYI, I do not see this in your v7 series.

--
Pranith