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 72E714156E8; Mon, 10 Aug 2026 15:44:58 +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=1786376701; cv=none; b=r1qFK+DbT+i9S6N/QomahK1gg86hVKUpyML/pJtWM1iTAjOze+OL8704wUd9coHYMF8kUSwI/Dj24lXufXCpAYcfjt/+hC6uWGORJG5B/Q2Z06fMUYpgtoTGyc9yvhDypTdwgvdhBZfdvKnvFGW6Ii1kNwStqgaLVD8HU010O+c= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786376701; c=relaxed/simple; bh=yQJww7ZGdYxbBgqIq9LKqYsedFQNlIaK8p+eVX/QSMM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=TI0Z/+lQrzh3NC/g+Wctyn90vqlq9gw0SffMqtBr7wr+7OKcg6nzuOEcZ4SMDki3DQiBfFtwDbB0Kor0llqqM9eIGhOLX0Uni9YiMcZCzt2BbIxjCGBkSrnF3aJOMRTj98aaMflokeIP/9TUN9kA6Eus03BClIklo9cNewmKwVE= 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=H7xR0Sxb; 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="H7xR0Sxb" 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 8F24814BF; Mon, 10 Aug 2026 08:44:53 -0700 (PDT) Received: from arm.com (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E89943F632; Mon, 10 Aug 2026 08:44:53 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1786376697; bh=yQJww7ZGdYxbBgqIq9LKqYsedFQNlIaK8p+eVX/QSMM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=H7xR0SxbusLzG7Sb2NlewiR4LBLzuyCgcVaKV/mKoQPpnR6bHNwkOyVwOWyGpfp4k d7shVtjki8k5SijDUSvkaCBaPFYfjWglGLvWzmgSD/R+NCvdMcdAMlGjf4JUDx7ab2 oXZo8hi566R+Gne4NSKq2YKsMKBRKFhYIG8CsXKs= Date: Mon, 10 Aug 2026 16:44:51 +0100 From: Catalin Marinas To: Will Deacon Cc: Jason Gunthorpe , Marek Szyprowski , "Aneesh Kumar K.V" , Mostafa Saleh , iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, Robin Murphy , Marc Zyngier , Steven Price , Suzuki K Poulose , Jiri Pirko , Petr Tesarik , Alexey Kardashevskiy , Dan Williams , Xu Yilun , linuxppc-dev@lists.ozlabs.org, linux-s390@vger.kernel.org, Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , Christophe Leroy , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , x86@kernel.org Subject: Re: [PATCH] arm64: swiotlb: Keep the default size for protected guests Message-ID: References: <20260807182017.GH200537@ziepe.ca> <7d22482f-d5fd-4cf4-9d32-6a54e02f2fbb@samsung.com> <28442500-1d53-4776-bae5-459c3ffc1cc6@samsung.com> <20260810130818.GN200537@ziepe.ca> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Mon, Aug 10, 2026 at 03:14:47PM +0100, Will Deacon wrote: > On Mon, Aug 10, 2026 at 10:08:18AM -0300, Jason Gunthorpe wrote: > > On Mon, Aug 10, 2026 at 12:46:51PM +0100, Will Deacon wrote: > > > On Mon, Aug 10, 2026 at 01:37:11PM +0200, Marek Szyprowski wrote: > > > > On 10.08.2026 12:20, Will Deacon wrote: > > > > > On Mon, Aug 10, 2026 at 02:59:42PM +0530, Aneesh Kumar K.V wrote: > > > > >> Marek Szyprowski writes: > > > > >> > > > > >>> On 07.08.2026 20:20, Jason Gunthorpe wrote: > > > > >>>> On Fri, Aug 07, 2026 at 06:13:49PM +0000, Mostafa Saleh wrote: > > > > >>>>> But the whole thing is best effort anyway, the kernel picks > > > > >>>>> IO_TLB_DEFAULT_SIZE which does not depend on the system topology or > > > > >>>>> how many devices or how much DMA they do. > > > > >>>>> SWIOTLB memory is wasted if unused so we should be careful around > > > > >>>>> that as it would be the other way around and users would have to > > > > >>>>> decrease it manually. > > > > >>>> Yeah, it is why the arch code shouldn't really be sizing it directly, > > > > >>>> it should be done in common code and, yes, we are probably going to > > > > >>>> have to do something alot smarter to have the common code better > > > > >>>> auto-tune this for the CC case.. > > > > >>> What about the $subject patch? I assume that it is still needed to > > > > >>> > > > > >>> restore the behavior that was altered by the "[PATCH v8 00/23] > > > > >>> > > > > >>> dma-mapping: Track shared DMA state through direct, pool and swiotlb > > > > >>> > > > > >>> paths?" patchset? > > > > >>> > > > > >> I would request that we pick this patch to fix the regression described > > > > >> in https://lore.kernel.org/all/yq5azeyxyfol.fsf@kernel.org/. > > > > > I really don't think we need it. CCA hardware isn't exactly widespread > > > > > and the KVM host side patches don't appear close to being merged. > > > > > > > > Does this mean that the branch for-next/coco [1] won't go to v7.3-rc1? > > > > Let's not make progress on guest support contingent on KVM CCA host > > side patches please. I expect the CSPs will have VM instance types > > available based on CCA within quarters, and Linux as a Guest should > > work in those environments regardless of what KVM is doing. I don't > > really expect full KVM support for years, frankly, the patchset is > > massive. Even Intel and AMD don't have full KVM support yet. > > To be clear: the only part I'm pushing back on is the realm-specific hack > to size the SWIOTLB area in arch/arm64/. Even with that hack applied, I > don't believe a one-size-fits-all value is going to work for everybody, > so it was really great to see that special-case removed in: > > https://lore.kernel.org/all/20260717180442.110954-18-aneesh.kumar@kernel.org/ We had Mostafa's patch here: https://lore.kernel.org/all/20260603110522.3331819-4-smostafa@google.com/ adding the same swiotlb buffer allocation for pKVM guests and realms. Aneesh's cleanup of SWIOTLB_FORCE inadvertently removed the 'else' clause and limited_addressing is now always checked, meaning that we get size limiting irrespective of whether we still need a default swiotlb. Normally I would consider this a regression on top of mainline and Mostafa's patches rather than a cleanup. However, if the default size of the bounce buffer was never sufficient for realms, as you said, we may need something better here anyway, so not worth fixing. I can see x86 in mem_encrypt_setup_arch() adjusting the bounce buffer to 6% of the guest memory. We could make up similar logic for arm64 or we could just rely on CONFIG_SWIOTLB_DYNAMIC (default off currently for arm64 I think). -- Catalin