Received-SPF: pass (google.com: domain of linux-kernel+bounces-194088-linux.lists.archive=gmail.com@vger.kernel.org designates 139.178.88.99 as permitted sender) client-ip=139.178.88.99;
Feedback-ID: i56a14606:Fastmail
User-Agent: Cyrus-JMAP/3.11.0-alpha0-491-g033e30d24-fm-20240520.001-g033e30d2
Precedence: bulk
MIME-Version: 1.0
Message-Id: <8ff9bc52-bf2f-4856-9335-14bf659e7e4c@app.fastmail.com>
In-Reply-To: <ZlcODqVXTDh6n0h-@J2N7QTR9R3>
References: <20240528120844.3523915-1-arnd@kernel.org>
 <ZlcODqVXTDh6n0h-@J2N7QTR9R3>
Date: Wed, 29 May 2024 14:29:37 +0200
From: "Arnd Bergmann" <arnd@arndb.de>
To: "Mark Rutland" <mark.rutland@arm.com>, "Arnd Bergmann" <arnd@kernel.org>
Cc: "Catalin Marinas" <catalin.marinas@arm.com>,
 "Will Deacon" <will@kernel.org>, "Jason Gunthorpe" <jgg@ziepe.ca>,
 "Valentin Schneider" <vschneid@redhat.com>, "Baoquan He" <bhe@redhat.com>,
 "Peter Zijlstra" <peterz@infradead.org>,
 linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] arm64/io: add constant-argument check
Content-Type: text/plain

On Wed, May 29, 2024, at 13:14, Mark Rutland wrote:
>>  {
>> -	if (count == 8 || count == 4 || count == 2 || count == 1) {
>> +	if (__builtin_constant_p(count) &&
>> +	    (count == 8 || count == 4 || count == 2 || count == 1)) {
>>  		__const_memcpy_toio_aligned32(to, from, count);
>>  		dgh();
>>  	} else {
>
> I don't think this is the right fix.
>
> The idea was that this was checked in __iowrite32_copy(), which does:
>
> 	#define __iowrite32_copy(to, from, count)                  \
> 	        (__builtin_constant_p(count) ?                     \
> 	                 __const_iowrite32_copy(to, from, count) : \
> 	                 __iowrite32_copy_full(to, from, count))
>
> ... and so __const_iowrite32_copy() should really be marked as __always_inline,
> and the same for __const_memcpy_toio_aligned32(), to guarantee that both get
> inlined and such that __const_memcpy_toio_aligned32() sees a constant.
>
> The same reasoning applies to __const_iowrite64_copy() and
> __const_memcpy_toio_aligned64().
>
> Checking for a constant in __const_iowrite32_copy() doesn't guarantee
> that __const_memcpy_toio_aligned32() is inlined and will actually see a
> constant.
>
> Does diff the below you for you?

Yes, your version addresses both failures I ran into, and
I think all other theoretical cases.

I would prefer to combine both though, using __always_inline
to force the compiler to pick the inline version over
__iowrite32_copy_full() even when it is optimizing for size
and it decides the inline version is larger, but removing
the extra complexity from the macro.

According to Jason, he used a macro here to be sure
that the compiler can detect an inline function argument
as constant when the value is known but it is not
a constant value according to the C standard.

This was indeed a problem in older versions of clang
that missed a lot of optimizations in the kernel, but
clang-8 and higher were changed to have the same behavior
as gcc here, so it is no longer necessary now that the
older versions are unable to build kernels.

     Arnd