Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:8 as permitted sender) client-ip=2620:137:e000::3:8;
Message-ID: <5dbd5717-1107-4a21-b166-995630499607@arm.com>
Date:   Mon, 11 Dec 2023 15:37:55 +0000
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 <will@kernel.org>
Cc:     Joerg Roedel <joro@8bytes.org>, Christoph Hellwig <hch@lst.de>,
        Vineet Gupta <vgupta@kernel.org>,
        Russell King <linux@armlinux.org.uk>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Huacai Chen <chenhuacai@kernel.org>,
        WANG Xuerui <kernel@xen0n.name>,
        Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Hanjun Guo <guohanjun@huawei.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Wei Liu <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
        Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Niklas Schnelle <schnelle@linux.ibm.com>,
        Matthew Rosato <mjrosato@linux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Rob Herring <robh+dt@kernel.org>,
        Frank Rowand <frowand.list@gmail.com>,
        Marek Szyprowski <m.szyprowski@samsung.com>,
        Jason Gunthorpe <jgg@ziepe.ca>, 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: <cover.1701268753.git.robin.murphy@arm.com>
 <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 <robin.murphy@arm.com>
In-Reply-To: <20231211153023.GA26048@willie-the-truck>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit
Precedence: bulk

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 <robin.murphy@arm.com>
>>>> ---
>>>>    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<<ncomp->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.