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From: Marc Zyngier <maz@kernel.org>
To: Gavin Shan <gshan@redhat.com>
Cc: will@kernel.org, kvmarm@lists.cs.columbia.edu,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled
Date: Mon, 26 Oct 2020 08:53:21 +0000	[thread overview]
Message-ID: <a676f540d19fba2468fd1a801948826a@kernel.org> (raw)
In-Reply-To: <333451bd-4730-4ebb-f76c-28fa5d0e1f7d@redhat.com>

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.

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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WARNING: multiple messages have this Message-ID (diff)
From: Marc Zyngier <maz@kernel.org>
To: Gavin Shan <gshan@redhat.com>
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: Mon, 26 Oct 2020 08:53:21 +0000	[thread overview]
Message-ID: <a676f540d19fba2468fd1a801948826a@kernel.org> (raw)
In-Reply-To: <333451bd-4730-4ebb-f76c-28fa5d0e1f7d@redhat.com>

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.

Thanks,

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

  parent reply	other threads:[~2020-10-26  8:53 UTC|newest]

Thread overview: 28+ 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 ` Gavin Shan
2020-10-25  0:27 ` [PATCH 1/3] KVM: arm64: Check if 52-bits PA is enabled Gavin Shan
2020-10-25  0:27   ` Gavin Shan
2020-10-25  9:52   ` Marc Zyngier
2020-10-25  9:52     ` Marc Zyngier
2020-10-25 22:23     ` Gavin Shan
2020-10-25 22:23       ` Gavin Shan
2020-10-26  8:40       ` Will Deacon
2020-10-26  8:40         ` Will Deacon
2020-10-26  8:53       ` Marc Zyngier [this message]
2020-10-26  8:53         ` Marc Zyngier
2020-10-26 22:48         ` Gavin Shan
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  0:27   ` Gavin Shan
2020-10-25 10:05   ` Marc Zyngier
2020-10-25 10:05     ` Marc Zyngier
2020-10-25 22:27     ` Gavin Shan
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  0:27   ` Gavin Shan
2020-10-25 10:48   ` Marc Zyngier
2020-10-25 10:48     ` Marc Zyngier
2020-10-25 23:04     ` Gavin Shan
2020-10-25 23:04       ` Gavin Shan
2020-10-26  8:55       ` Marc Zyngier
2020-10-26  8:55         ` Marc Zyngier

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