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

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Date:   Mon, 26 Oct 2020 08:40:43 +0000
From:   Will Deacon <will@...nel.org>
To:     Gavin Shan <gshan@...hat.com>
Cc:     Marc Zyngier <maz@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, alexandru.elisei@....com
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled

On Mon, Oct 26, 2020 at 09:23:31AM +1100, Gavin Shan wrote:
> 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.

I don't know: adding #ifdef CONFIG_ lines just reduces the coverage we
get from CI, so unless the code is actually causing a problem then I'd be
inclined to leave it as-is.

Will