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 8DE253EC82C; Mon, 10 Aug 2026 14:14:57 +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=1786371298; cv=none; b=EQs/CE3U0IIw5hfUHVSzV083TvEfRFhWeJqWmKXK/RNyGi/iRRsncOjryI7E6GMm4NOhtOK7zWcPVvLXvX9EIzsJcvJmldWC48CNfzjprCH/3/8a/fsRHiyYvxBNZfQz/eyEdrOtJ3sAktOVkCCdDbmDALEyRxcCeh+LvSR0fp0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786371298; c=relaxed/simple; bh=f0KjkiUVLIfdRbBzyOjBy2QoB9diSFrEw4kN60BEQLI=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=G6O1sHQSQ1y3AXKFyFTHnYSgAjjrp0m05rLMP+t1rR3/15ztRW5PHwy9JQWRZzh/jPqa2KL8Wz5tvYiiOouyrd2WI10a8zWg9brhsa0YSIvR+VBJqM/+wnrXy7MWGEH6sYsoDEoAOdPI6jJOdGJz247TCkywphSRIVqhDhqBq3M= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=FBkTu6+8; 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="FBkTu6+8" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8E94C1F000E9; Mon, 10 Aug 2026 14:14:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786371296; bh=Cqbq7ocjp5qLqhn6ub1Jz2ULUPz57xxAkQlmliZcS0c=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=FBkTu6+85usDU52Dha/5DQXVxWOENVS7yKDU9Ofdw3/oCPIg8ZLLUDSMcKrnVSP9L f3c2cikYrYuD545fo9Hujadnw/NDojGZoRhVIg9Jw3Posb1ONG7re4pg3rkKq4FDSi +veUdAwky9ShQkN2t7ZmYRCR2su1bedXoyHeAfhfydhcQAYdnKuNH0K4+OncmDgbXR c0VrUqERtZN0kZ8GV6c/ch6F0MNItNaGkFJrLo0roy86L4K6/lG4uLnvSgbdwglunP MNHEtAN3z1+wuwpt11fYQGr3o9Sd77LRs1gJTjf1yjJVk7tzduLbddd70pQP+LLyoY mmQse+6xWKhIg== Date: Mon, 10 Aug 2026 15:14:47 +0100 From: Will Deacon To: Jason Gunthorpe Cc: 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 , Catalin Marinas , 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: <20260807164734.GE200537@ziepe.ca> <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: <20260810130818.GN200537@ziepe.ca> 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/ but now, because it regresses some test configuration, the proposal was to penalise protected VMs too (bearing in mind that the patch above hasn't yet landed upstream): https://lore.kernel.org/all/20260807092612.2202005-1-aneesh.kumar@kernel.org/ with the alternative being to reintroduce the original hack that we just removed! https://lore.kernel.org/all/yq5acxvu2ict.fsf@kernel.org/ I'm saying: merge the patches as they are, without reintroducing this horrible bodge in the architecture code to drive a heuristic that most people seem to agree should be handled more robustly elsewhere. > People already have CCA capable HW, are already testing this stuff and > the closer upstream can get to being workable as a guest without a > mountain of OOT patches the better. This specific part is about a single line of code, to control something which is already configurable in Kconfig and on the cmdline. It's not a mountain of out-of-tree patches. > I agree the thing is not ideal, but it was merged to ARM like this a > long time ago, this patch is just fixing a small oopsie (was it a > merge conflict?) to put it back. I don't the objection. My main objection is that I have very little confidence in people trying to fix this properly if we take the realm-specific bodge in the arch code. With all the CCA patches floating about already, why would they? Will