From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 65EEE32E151; Mon, 10 Aug 2026 15:58:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786377531; cv=none; b=l6OcfVJuEunQ8zBJAMql53jPQZvWoPdWGdT/YsM/ecfvEln5TV30f5YiNmBimeo/s1WsioBIx2E706c0e/rbeNiqkAXrAVfmjuyAKaaV030yJd8uVMlPUc+j81oJJvvxNrYgBjRiUpeItjqgw5bDAUtIvtyNjD+N32gX3gEE5B8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786377531; c=relaxed/simple; bh=dMsXiko1vfrWhQ1Gvy2SuJ+MBDqh+CxBCDgQglqAxjw=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Gx8gtI4kWje21E63XE1mmDpl9wm8q3Bh038lYhgPArERTprGhQs7BLMe5xRDFc1FNWgj3MR/Xqd14Gl1Ayb36wRol+BoXG2FmFhaWKO2U5eu2Tc1lcMBdQWdj8guQmquMFITFZLb3IM0I+HDCAZp7USCLuHaXRQZdbiVox6pWH8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=IhYR6wGF; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IhYR6wGF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D32581F000E9; Mon, 10 Aug 2026 15:58:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786377530; bh=JNujZOp6SvxQUqxxQAnbQ1M+55BGiKjS96+FFU7yPIw=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=IhYR6wGF3mEiQ3fkJyZhchwlWwIuqXxVgyx97z/WA5KqRyecBvU6zy0Hg6lpL6SYc QYBTi9wWbkz2DzQw8TXn15QrwGtypcBN8ddGE/Tmw2xZabRVlmCPnVn3Pt9SCQ3iMU ZEuaVfhKzSumYrsNU8o+wPC7nO0Bf+UYG2VwfktxmZieTUBvDcQu6G3ZvJXyjbmnz4 oZ91MIsniybaPNFpcM00hW3/IkLRXu+8tSKD0ph/X9SR8iMO4R0qfJrl3QX5gC6mcC A4uZ0ogkLO08jyHFqegDcY3/L7mDflUgMhXn27rszSsNTXcJoGL9n6J0zvvHOF8rHg QPolLhvuE/uSg== Date: Mon, 10 Aug 2026 16:58:42 +0100 From: Will Deacon To: Catalin Marinas 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: linux-coco@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 04:44:51PM +0100, Catalin Marinas wrote: > 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. Yes, and I think that's bad for pKVM guests which is why I'm happy to see that part removed. > 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. Inadvertently, perhaps, but it's got Tested-bys from three different companies on it and I think it's a good change. I don't think we should be special-casing these environments based on some random collection of virtio devices in a testing setup. > 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). As Jason said, this should be done outside the arch code. Will