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 03DAB3469FC; Wed, 19 Aug 2026 13:05:42 +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=1787144745; cv=none; b=kdjNxo7L0aOFn/g4hpufTl8yluDpRK49GnWE2mjLxsHLsR4bbDCYEEEz8TZ67dVAF2Sya8tkJJh/B3taBE3sZJl4c2rwZXgGOFJFKx26VqkRayAyqh637E9aV0zJYW9qGXvViVJ8nUsJlUFb3X2mNe8I2xcczbA9HV0H0qR27Xc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1787144745; c=relaxed/simple; bh=8tZeoA1qM6yseHc5wH6x5IAXCz7o0dIu8IBtXCtDR5I=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=etAHTbBH5EO7kNlx2W/4a5ewRuKwsx76OJm0olw98iJG/pDWa4NKib9YOV6o50gCYUfmAuGfbU+dSvLc+pxTXLF8zJfnqIo3y3EdcpTP3ZemQmm8JePAgO6JOdKGXQs4mCe4Q9BOFReWy5cPlRyUZBFDaN4nm6+XQaILJvfmO1Q= 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=t3yWksBP; 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="t3yWksBP" 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 DAF8214BF; Wed, 19 Aug 2026 06:05:31 -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 4142F3F763; Wed, 19 Aug 2026 06:05:32 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1787144735; bh=8tZeoA1qM6yseHc5wH6x5IAXCz7o0dIu8IBtXCtDR5I=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=t3yWksBPwJHtK3Fh45zd2qMesmCsZj0EcT0Z3hkyOVtVEG9Mxa6bfiidSqLWTXzo4 whdeeirC8wqKVHU+1MTFT5sLwfrgQ2XXTsu9lig87KYPcNaBdKnnKT+HDpBjIv1tHd 6Sq3qDJKVnxymVmnOpi18PnEp+N8VXvbXJBYRKRY= Date: Wed, 19 Aug 2026 14:05:29 +0100 From: Catalin Marinas To: "Aneesh Kumar K.V" Cc: iommu@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-coco@lists.linux.dev, Robin Murphy , Marek Szyprowski , Will Deacon , Marc Zyngier , Steven Price , Suzuki K Poulose , Jiri Pirko , Jason Gunthorpe , Mostafa Saleh , Petr Tesarik , Alexey Kardashevskiy , 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: [RFC PATCH v2 2/2] dma: swiotlb: Initialize and size shared default pools for memory encryption Message-ID: References: <20260813102521.1367737-1-aneesh.kumar@kernel.org> <20260813102521.1367737-3-aneesh.kumar@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Wed, Aug 19, 2026 at 05:28:03PM +0530, Aneesh Kumar K.V wrote: > Catalin Marinas writes: > > On Thu, Aug 13, 2026 at 03:55:21PM +0530, Aneesh Kumar K.V (Arm) wrote: > >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c > >> index 9f5b366d2086..c3188ca878f3 100644 > >> --- a/arch/arm64/mm/init.c > >> +++ b/arch/arm64/mm/init.c > >> @@ -338,12 +338,8 @@ void __init arch_setup_zero_pages(void) > >> void __init arch_mm_preinit(void) > >> { > >> unsigned int flags = SWIOTLB_VERBOSE; > >> - /* pKVM uses restricted-dma-pool */ > >> - bool cc_guest = is_realm_world(); > >> > >> - if (cc_guest) > >> - flags |= SWIOTLB_INIT_CC_SHARED; > >> - else if (max_pfn > PFN_DOWN(arm64_dma_phys_limit)) > >> + if (max_pfn > PFN_DOWN(arm64_dma_phys_limit)) > >> flags |= SWIOTLB_INIT_ADDRESSING_LIMIT; > >> > >> swiotlb_init(flags); > > > > This looks fine. As I mentioned on patch 1, we might as well move this > > hunk over there and avoid the flag definition. > > > >> @@ -102,9 +101,6 @@ void __init mem_encrypt_init(void) > >> > >> void __init mem_encrypt_setup_arch(void) > >> { > >> - phys_addr_t total_mem = memblock_phys_mem_size(); > >> - unsigned long size; > >> - > >> /* > >> * Do RMP table fixups after the e820 tables have been setup by > >> * e820__memory_setup(). > >> @@ -112,33 +108,9 @@ void __init mem_encrypt_setup_arch(void) > >> if (cc_platform_has(CC_ATTR_HOST_SEV_SNP)) > >> snp_fixup_e820_tables(); > >> > >> - if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) > >> - x86_swiotlb_flags |= SWIOTLB_INIT_CC_SHARED; > >> - > >> if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) > >> return; > >> > >> - /* > >> - * For SEV and TDX, all DMA has to occur via shared/unencrypted pages. > >> - * Kernel uses SWIOTLB to make this happen without changing device > >> - * drivers. However, depending on the workload being run, the > >> - * default 64MB of SWIOTLB may not be enough and SWIOTLB may > >> - * run out of buffers for DMA, resulting in I/O errors and/or > >> - * performance degradation especially with high I/O workloads. > >> - * > >> - * Adjust the default size of SWIOTLB using a percentage of guest > >> - * memory for SWIOTLB buffers. Also, as the SWIOTLB bounce buffer > >> - * memory is allocated from low memory, ensure that the adjusted size > >> - * is within the limits of low available memory. > >> - * > >> - * The percentage of guest memory used here for SWIOTLB buffers > >> - * is more of an approximation of the static adjustment which > >> - * 64MB for <1G, and ~128M to 256M for 1G-to-4G, i.e., the 6% > >> - */ > >> - size = total_mem * 6 / 100; > >> - size = clamp_val(size, IO_TLB_DEFAULT_SIZE, SZ_1G); > >> - swiotlb_adjust_size(size); > >> - > >> /* Set restricted memory access for virtio. */ > >> virtio_set_mem_acc_cb(virtio_require_restricted_mem_acc); > >> } > > > > Credit to claude, it noticed a slight change in behaviour for x86 w.r.t. > > the crash kernel reservation. crash_low_size_default() reads the swiotlb > > size but the resizing now happens after arch_reserve_crashkernel(). > > Maybe not an issue. > > > > Alternatively, we could build the sizing logic into > > swiotlb_size_or_default() but I haven't checked whether we have the > > right information when this function is called. > > IIUC, the current code can still get a different value from > crash_low_size_default() than the final swiotlb size we end up using. > This is because crash_low_size_default() is computed early, before > default_nareas, which is derived from num_possible_cpus(), has been set. > > If we are okay with keeping this consistent with the existing behavior, > moving sizing logic to swiotlb_adjusted_size() looks like a clean option. See how it looks, it may turn out cleaner. > > I think at a high level, we need (i.e. separate attributed from sizing): > > > > if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) > > io_tlb_default_mem.cc_shared = true; > > > > if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) { > > if (!restricted_dma_pool_present) > > swiotlb_adjust_cc_size(); /* 6%, clamped */ > > } else if (!(flags & SWIOTLB_INIT_ADDRESSING_LIMIT) && > > swiotlb_kmalloc_needs_bounce()) { > > swiotlb_shrink_for_kmalloc(); /* 1MB per 1GB */ > > } > > > > But pKVM wants to reduce the swiotlb size based on > kmalloc_needs_bounce() when it is using a restricted-dma-pool. > > ie, > if (cc_platform_has(CC_ATTR_MEM_ENCRYPT)) > io_tlb_default_mem.cc_shared = true; > .. > > if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && > !restricted_dma_pool_present) { > swiotlb_adjust_cc_attributes(); > } else if (!(flags & SWIOTLB_INIT_ADDRESSING_LIMIT) && > swiotlb_kmalloc_needs_bounce()) { Ah, right, I got my conditions wrong. Basically a restricted pool prevents an swiotlb growth in a protected guest. If no addressing limit but kmalloc bouncing, we allocate a small one. Otherwise we should not have any swiotlb at all. I need to write it in a table, too many combinations CC guest: no restricted pool => 6% of RAM restricted pool (pKVM heuristics): addressing limit => default size no addressing limit: kmalloc needs bounce => minimal no kmalloc bouncing => no default swiotlb Host CC_ATTR_MEM_ENCRYPT => default size No CC: addressing limit => default size no addressing limit: kmalloc needs bounce => minimal no kmalloc bouncing => no default swiotlb -- Catalin