Re: [PATCH] KVM: Switch 'requests' to be 64-bit (explicitly)

[Paolo Bonzini]

On 22/05/2018 17:42, Raslan, KarimAllah wrote:
> On Mon, 2018-04-16 at 18:28 +0200, Paolo Bonzini wrote:
>> On 15/04/2018 00:26, KarimAllah Ahmed wrote:
>>>
>>> Switch 'requests' to be explicitly 64-bit and update BUILD_BUG_ON check to
>>> use the size of "requests" instead of the hard-coded '32'.
>>>
>>> That gives us a bit more room again for arch-specific requests as we
>>> already ran out of space for x86 due to the hard-coded check.
>>>
>>> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
>>> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
>>> Cc: kvm@xxxxxxxxxxxxxxx
>>> Cc: linux-kernel@xxxxxxxxxxxxxxx
>>> Signed-off-by: KarimAllah Ahmed <karahmed@xxxxxxxxx>
>>
>> I'm afraid architectures like ARM 32 need this to be conditional (using
>> Kconfig).
> 
> Why would using a 64-bit 'requests' be a problem for ARM32? Are youÂ
> concerned about performance here or is there some symantic problem?

They don't support atomics on double-word data.

Paolo