From: Will Deacon <will@kernel.org>
To: Gavin Shan <gshan@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>,
kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
alexandru.elisei@arm.com
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled
Date: Mon, 26 Oct 2020 08:40:43 +0000 [thread overview]
Message-ID: <20201026084042.GC23739@willie-the-truck> (raw)
In-Reply-To: <333451bd-4730-4ebb-f76c-28fa5d0e1f7d@redhat.com>
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@redhat.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
next prev parent reply other threads:[~2020-10-26 8:40 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-25 0:27 [PATCH 0/3] KVM: arm64: Failback on unsupported huge pages Gavin Shan
2020-10-25 0:27 ` [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled Gavin Shan
2020-10-25 9:52 ` Marc Zyngier
2020-10-25 22:23 ` Gavin Shan
2020-10-26 8:40 ` Will Deacon [this message]
2020-10-26 8:53 ` Marc Zyngier
2020-10-26 22:48 ` Gavin Shan
2020-10-25 0:27 ` [PATCH 2/3] KVM: arm64: Don't map PUD huge page if it's not available Gavin Shan
2020-10-25 10:05 ` Marc Zyngier
2020-10-25 22:27 ` Gavin Shan
2020-10-25 0:27 ` [PATCH 3/3] KVM: arm64: Failback on unsupported huge page sizes Gavin Shan
2020-10-25 10:48 ` Marc Zyngier
2020-10-25 23:04 ` Gavin Shan
2020-10-26 8:55 ` Marc Zyngier
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