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

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Date:   Mon, 26 Oct 2020 08:53:21 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
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 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.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...