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 9FD5B526AA4; Wed, 23 Sep 2026 13:04:26 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790168668; cv=none; b=YlNAKtnZDCmi2SaowKC1qY8Cck7gmPwalZkW/u8JbgJKqeR/xVWDiWkzLSwV6RE7dNK+G2q0WpGS1X0efZVt3C4c7emCiVajI9UjPrHAFmo5ZMvqblfxAIY3P8qneQNVr5j4is8DJmEbim8py5wM+z9pUDF6Zydxxfqr4zmAy6Y= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1790168668; c=relaxed/simple; bh=V2eG6fezUymbQQ69ZuSww8KCP/5bN/QHpNMXTsYSPOw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fE8jdQcgaamktJYs+Fh0DfQ2jyc00UJpQxxsfsn75cuZs7AjO6hrrNS01ncSgje+0ns1Hk93YSZ8647xvRXlsHIsDr1Cd9hbiYwfGmW+ofXUBvnxnvPBacCYrMkL2FwvQOJePq0x3ZdQGkggTzFr5nZo5QwBWnxeBacdise1h8c= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=ZvqzetxD; arc=none smtp.client-ip=217.140.110.172 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 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="ZvqzetxD" 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 6A0231570; Wed, 23 Sep 2026 06:04:22 -0700 (PDT) Received: from [10.2.212.23] (e121345-lin.cambridge.arm.com [10.2.212.23]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A1A663F86F; Wed, 23 Sep 2026 06:04:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1790168665; bh=V2eG6fezUymbQQ69ZuSww8KCP/5bN/QHpNMXTsYSPOw=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=ZvqzetxDsEV+z9y8tgXSp1WMJmVGbPD4FFevEiSlqja3DG2IGWuF2b73OrQ3DI5PP ebwj8SPEQZ4BaUOAgvsq/NUWIH7HsMxifQ4aW+kmBXBfzWreFCdx8DwnXmCnT2MCb1 pQF5HqiW/0gA6vvYspF/vjHUh2HXW5nKxCvWawCQ= Message-ID: <8c9c04ae-cba7-4096-a7d1-70b90a46c161@arm.com> Date: Wed, 23 Sep 2026 14:04:00 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 5/6] dma: swiotlb: Add an overridable architecture pool opt-out To: "Aneesh Kumar K.V (Arm)" , iommu@lists.linux.dev, linux-kernel@vger.kernel.org Cc: Marek Szyprowski , Will Deacon , Marc Zyngier , Steven Price , Suzuki K Poulose , Catalin Marinas , Jiri Pirko , Jason Gunthorpe , Mostafa Saleh , Petr Tesarik , Alexey Kardashevskiy , Dan Williams , Xu Yilun , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , Russell King , Huacai Chen , Thomas Bogendoerfer , Jiaxun Yang , Paul Walmsley , Palmer Dabbelt , Albert Ou , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, x86@kernel.org References: <20260921063628.362078-1-aneesh.kumar@kernel.org> <20260921063628.362078-6-aneesh.kumar@kernel.org> From: Robin Murphy Content-Language: en-GB In-Reply-To: <20260921063628.362078-6-aneesh.kumar@kernel.org> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 21/09/2026 7:36 am, Aneesh Kumar K.V (Arm) wrote: > On x86, iommu=off historically prevented SWIOTLB initialization by not > setting SWIOTLB_INIT_ADDRESSING_LIMIT. This overloads that flag: it > controls both whether a pool is selected and where the pool is > allocated. If another requirement, swiotlb=force selects a pool > independently, omitting SWIOTLB_INIT_ADDRESSING_LIMIT may place it above > the addressable range of devices which need bounce buffering. Well then surely something is wrong with the design? If SWIOTLB_INIT_ADDRESSING_LIMIT is set then SWIOTLB still needs to respect it irrespective of any *additional* reasons for activating (either from arch code or internally). And conversely, if devices have addressing limits but SWIOTLB_INIT_ADDRESSING_LIMIT is *not* set, then the arch code has clearly gone wrong or made a bad assumption, but that doesn't seem like SWIOTLB's problem to worry about. > Add SWIOTLB_INIT_FORCE_DISABLE to let an architecture opt out of normal > pool initialization without discarding the pool placement constraints. > > Unlike swiotlb_force_disable, which is set by swiotlb=noforce and > unconditionally prevents pool initialization, the new flag is an > architecture default. Explicit requirements such as memory encryption, > architecture remapping, or swiotlb=force take precedence over it. Logically, that should already be implied by the architecture passing no SWIOTLB_INIT flags. Thanks, Robin. > iommu=soft retains SWIOTLB_INIT_ADDRESSING_LIMIT because it requests > software bounce buffering for devices whose DMA masks cannot address all > system memory. Such devices must be able to reach the bounce pool. > > Clear both flags for Xen because Xen explicitly requires a remapped > SWIOTLB pool and may allocate it anywhere in directly mapped memory. > > With no override, iommu=off disables both hardware IOMMU translation and > SWIOTLB bouncing. Direct DMA remains available; mappings outside a > device's addressable range fail instead of being bounced. > > Reviewed-by: Catalin Marinas > Signed-off-by: Aneesh Kumar K.V (Arm) > --- > arch/x86/kernel/pci-dma.c | 11 ++++++++--- > include/linux/swiotlb.h | 2 ++ > kernel/dma/swiotlb.c | 10 ++++++++-- > 3 files changed, 18 insertions(+), 5 deletions(-) > > diff --git a/arch/x86/kernel/pci-dma.c b/arch/x86/kernel/pci-dma.c > index 0cff255827ba..ce257b20ea9c 100644 > --- a/arch/x86/kernel/pci-dma.c > +++ b/arch/x86/kernel/pci-dma.c > @@ -44,9 +44,13 @@ static unsigned int x86_swiotlb_flags; > static void __init pci_swiotlb_detect(void) > { > /* don't initialize swiotlb if iommu=off (no_iommu=1) */ > - if (!no_iommu && max_possible_pfn > MAX_DMA32_PFN) { > - x86_swiotlb_enable = true; > + if (no_iommu) > + x86_swiotlb_flags |= SWIOTLB_INIT_DEFAULT_OFF; > + > + if (max_possible_pfn > MAX_DMA32_PFN) { > x86_swiotlb_flags |= SWIOTLB_INIT_ADDRESSING_LIMIT; > + if (!no_iommu) > + x86_swiotlb_enable = true; > } > > /* > @@ -84,7 +88,8 @@ static void __init pci_xen_swiotlb_init(void) > return; > x86_swiotlb_enable = true; > /* Xen can use a SWIOTLB pool anywhere in directly mapped memory. */ > - x86_swiotlb_flags &= ~SWIOTLB_INIT_ADDRESSING_LIMIT; > + x86_swiotlb_flags &= ~(SWIOTLB_INIT_ADDRESSING_LIMIT | > + SWIOTLB_INIT_DEFAULT_OFF); > x86_swiotlb_flags |= SWIOTLB_INIT_REMAP | SWIOTLB_ANY; > swiotlb_init_remap(x86_swiotlb_flags, xen_swiotlb_fixup); > dma_ops = &xen_swiotlb_dma_ops; > diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h > index be3962a33fc6..466c33795db0 100644 > --- a/include/linux/swiotlb.h > +++ b/include/linux/swiotlb.h > @@ -20,6 +20,8 @@ struct scatterlist; > #define SWIOTLB_INIT_ADDRESSING_LIMIT (1 << 2) > /* Initialize a default-sized pool that requires architecture remapping. */ > #define SWIOTLB_INIT_REMAP (1 << 3) > +/* Do not initialize a pool unless SWIOTLB is explicitly required. */ > +#define SWIOTLB_INIT_DEFAULT_OFF (1 << 4) > > /* > * Maximum allowable number of contiguous slabs to map, > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c > index c19675846fc1..46b789eb0806 100644 > --- a/kernel/dma/swiotlb.c > +++ b/kernel/dma/swiotlb.c > @@ -563,10 +563,16 @@ swiotlb_select_pool_policy(unsigned int flags) > if (flags & SWIOTLB_INIT_REMAP) > return SWIOTLB_POOL_DEFAULT; > > - if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT) > + if (swiotlb_force_bounce) > return SWIOTLB_POOL_DEFAULT; > > - if (swiotlb_force_bounce) > + /* > + * Explicit requirements above override an architecture's default opt-out. > + */ > + if (flags & SWIOTLB_INIT_DEFAULT_OFF) > + return SWIOTLB_POOL_NONE; > + > + if (flags & SWIOTLB_INIT_ADDRESSING_LIMIT) > return SWIOTLB_POOL_DEFAULT; > > if (swiotlb_kmalloc_needs_bounce())