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

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Message-ID: <5f346c4e-73f2-6a73-7915-e9fa911191ec@redhat.com>
Date:   Tue, 22 May 2018 17:47:42 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     "Raslan, KarimAllah" <karahmed@...zon.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "rkrcmar@...hat.com" <rkrcmar@...hat.com>
Subject: Re: [PATCH] KVM: Switch 'requests' to be 64-bit (explicitly)

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@...hat.com>
>>> Cc: Radim Krčmář <rkrcmar@...hat.com>
>>> Cc: kvm@...r.kernel.org
>>> Cc: linux-kernel@...r.kernel.org
>>> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
>>
>> 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 you 
> concerned about performance here or is there some symantic problem?

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

Paolo