From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 6C38D49F7F for ; Mon, 11 Dec 2023 15:38:02 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 7BE16FEC; Mon, 11 Dec 2023 07:38:48 -0800 (PST) Received: from [10.57.85.194] (unknown [10.57.85.194]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EF49C3F738; Mon, 11 Dec 2023 07:37:55 -0800 (PST) Message-ID: <5dbd5717-1107-4a21-b166-995630499607@arm.com> Date: Mon, 11 Dec 2023 15:37:55 +0000 Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 3/7] ACPI/IORT: Handle memory address size limits as limits Content-Language: en-GB To: Will Deacon Cc: Joerg Roedel , Christoph Hellwig , Vineet Gupta , Russell King , Catalin Marinas , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , Paul Walmsley , Palmer Dabbelt , Albert Ou , Lorenzo Pieralisi , Hanjun Guo , Sudeep Holla , "K. Y. Srinivasan" , Haiyang Zhang , Wei Liu , Dexuan Cui , Suravee Suthikulpanit , David Woodhouse , Lu Baolu , Niklas Schnelle , Matthew Rosato , Gerald Schaefer , Jean-Philippe Brucker , Rob Herring , Frank Rowand , Marek Szyprowski , Jason Gunthorpe , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-acpi@vger.kernel.org, iommu@lists.linux.dev, devicetree@vger.kernel.org References: <2ae6199a9cf035c1defd42e48675b827f41cdc95.1701268753.git.robin.murphy@arm.com> <20231211132757.GE25681@willie-the-truck> <91b22090-485f-49c9-a536-849fd7f92f8e@arm.com> <20231211153023.GA26048@willie-the-truck> From: Robin Murphy In-Reply-To: <20231211153023.GA26048@willie-the-truck> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2023-12-11 3:30 pm, Will Deacon wrote: > On Mon, Dec 11, 2023 at 03:01:27PM +0000, Robin Murphy wrote: >> On 2023-12-11 1:27 pm, Will Deacon wrote: >>> On Wed, Nov 29, 2023 at 05:43:00PM +0000, Robin Murphy wrote: >>>> Return the Root Complex/Named Component memory address size limit as an >>>> inclusive limit value, rather than an exclusive size. This saves us >>>> having to special-case 64-bit overflow, and simplifies our caller too. >>>> >>>> Signed-off-by: Robin Murphy >>>> --- >>>> drivers/acpi/arm64/dma.c | 9 +++------ >>>> drivers/acpi/arm64/iort.c | 18 ++++++++---------- >>>> include/linux/acpi_iort.h | 4 ++-- >>>> 3 files changed, 13 insertions(+), 18 deletions(-) >>> >>> [...] >>> >>>> diff --git a/drivers/acpi/arm64/iort.c b/drivers/acpi/arm64/iort.c >>>> index 6496ff5a6ba2..eb64d8e17dd1 100644 >>>> --- a/drivers/acpi/arm64/iort.c >>>> +++ b/drivers/acpi/arm64/iort.c >>>> @@ -1367,7 +1367,7 @@ int iort_iommu_configure_id(struct device *dev, const u32 *input_id) >>>> { return -ENODEV; } >>>> #endif >>>> -static int nc_dma_get_range(struct device *dev, u64 *size) >>>> +static int nc_dma_get_range(struct device *dev, u64 *limit) >>>> { >>>> struct acpi_iort_node *node; >>>> struct acpi_iort_named_component *ncomp; >>>> @@ -1384,13 +1384,12 @@ static int nc_dma_get_range(struct device *dev, u64 *size) >>>> return -EINVAL; >>>> } >>>> - *size = ncomp->memory_address_limit >= 64 ? U64_MAX : >>>> - 1ULL<memory_address_limit; >>>> + *limit = (1ULL << ncomp->memory_address_limit) - 1; >>> >>> The old code handled 'ncomp->memory_address_limit >= 64' -- why is it safe >>> to drop that? You mention it in the cover letter, so clearly I'm missing >>> something! >> >> Because an unsigned shift by 64 or more generates 0 (modulo 2^64), thus >> subtracting 1 results in the correct all-bits-set value for an inclusive >> 64-bit limit. > > Oh, I'd have thought you'd have gotten one of those "left shift count >= > width of type" warnings if you did that. Compilers might give such a warning if it was a constant shift whose size was visible at compile time, but even then only because compilers seem to have a vendetta against us relying on the well-defined behaviours of unsigned integer overflow (it's only *signed* shifts which are UB if the result is unrepresentable). Cheers, Robin.