Re: [PATCH] xen: Introduce cmpxchg64() and guest_cmpxchg64()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.08.2020 15:03, Julien Grall wrote:
> On 17/08/2020 12:50, Roger Pau Monné wrote:
>> On Mon, Aug 17, 2020 at 12:05:54PM +0100, Julien Grall wrote:
>>> The only way I could see to make it work would be to use the same trick as
>>> we do for {read, write}_atomic() (see asm-arm/atomic.h). We are using union
>>> and void pointer to prevent explicit cast.
>>
>> I'm mostly worried about common code having assumed that cmpxchg
>> does also handle 64bit sized parameters, and thus failing to use
>> cmpxchg64 when required. I assume this is not much of a deal as then
>> the Arm 32 build would fail, so it should be fairly easy to catch
>> those.
> FWIW, this is not very different to the existing approach. If one would 
> use cmpxchg() with 64-bit, then it would fail to compile.

A somewhat related question then: Do you really need both the
guest_* and the non-guest variants? Limiting things to plain
cmpxchg() would further reduce the risk of someone picking the
wrong one without right away noticing the build issue on Arm32.
For guest_cmpxchg{,64}() I think there's less of a risk.

Jan