linux-kernel - Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled

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Date:   Tue, 27 Oct 2020 09:48:01 +1100
From:   Gavin Shan <gshan@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        will@...nel.org, alexandru.elisei@....com
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled

On 10/26/20 7:53 PM, Marc Zyngier wrote:
> On 2020-10-25 22:23, Gavin Shan wrote:
>> Hi Marc,
>>
>> On 10/25/20 8:52 PM, Marc Zyngier wrote:
>>> On Sun, 25 Oct 2020 01:27:37 +0100,
>>> Gavin Shan <gshan@...hat.com> wrote:
>>>>
>>>> The 52-bits physical address is disabled until CONFIG_ARM64_PA_BITS_52
>>>> is chosen. This uses option for that check, to avoid the unconditional
>>>> check on PAGE_SHIFT in the hot path and thus save some CPU cycles.
>>>
>>> PAGE_SHIFT is known at compile time, and this code is dropped by the
>>> compiler if the selected page size is not 64K. This patch really only
>>> makes the code slightly less readable and the "CPU cycles" argument
>>> doesn't hold at all.
>>>
>>> So what are you trying to solve exactly?
>>>
>>
>> There are two points covered by the patch: (1) The 52-bits physical address
>> is visible only when CONFIG_ARM64_PA_BITS_52 is enabled in arch/arm64 code.
>> The code looks consistent with this option used here. (2) I had the assumption
>> that gcc doesn't optimize the code and PAGE_SHIFT is always checked in order
>> to get higher 4 physical address bits, but you said gcc should optimize the
>> code accordingly. However, it would be still nice to make the code explicit.
> 
> Conditional compilation only results in more breakages, specially for configs
> that hardly anyone uses (big-endian and 64K pages are the two that bitrot very
> quickly).
> 
> So if anything can build without #ifdef, I'll take that anytime. If the compiler
> doesn't optimize it away, let's fix the compiler.
> 

Ok. PATCH[1] and PATCH[2] have been dropped in v2.

Cheers,
Gavin