From: Gavin Shan <gshan@redhat.com>
To: Marc Zyngier <maz@kernel.org>
Cc: kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org,
will@kernel.org, alexandru.elisei@arm.com
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled
Date: Tue, 27 Oct 2020 09:48:01 +1100 [thread overview]
Message-ID: <11639356-397a-4a25-879a-0c91d9845041@redhat.com> (raw)
In-Reply-To: <a676f540d19fba2468fd1a801948826a@kernel.org>
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@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.
>
> 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
next prev parent reply other threads:[~2020-10-26 22:48 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
2020-10-26 8:53 ` Marc Zyngier
2020-10-26 22:48 ` Gavin Shan [this message]
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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