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 DF1D036E466; Fri, 7 Aug 2026 18:00:08 +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=1786125610; cv=none; b=U1SmjIMSZTq5usqLsCrYSejH0TFoopvDKcXLZZs/OeEVvGqySTrN8uBefnlz0KTrWMiSZr1MFnaGJizPcN2Qh7NFuBzW+k5v3pPyUXS4pvdRR8lQlC81s0QlwsnhARVH4K+m3bT8iMEUDOSsNr9NbtUstlJfEv5do4BUJns08Rs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786125610; c=relaxed/simple; bh=1Oe182UghJB6zmvPQ4B5TKJJr3BW4MdOLnWvw8BiJ34=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=DrhC1bBpbDOiWYt4kY08YUgWwh3BBqORl7xT+bK6lci+e4fpFfiGwlUj+g5ttcdp9bkG6echjIUmoTcQKMivTyO9ZSyN817PLb9Eg1+OOvi2n5rXzMk2lOVu4U/oqX6U6X1dJ1ffLhtnlSkl7abRdtrG79Kc8Yd7i1R1wammPE4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=kJD5BuIF; 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="kJD5BuIF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 113191F000E9; Fri, 7 Aug 2026 17:59:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786125608; bh=anTn3vLV1JrxTHR3F12/3ps8riKNqYfPGpQ3iHTztQc=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=kJD5BuIFAjMSJG3vrvonufIfl5PkjhSvEyRTNa2e+eKbjs9hpZsRN0aT/dRE5qzcG m6/ZymFBQPJ706AeGlXg1d3uSKhWNEQ8Nd+p6g8bsUaPTzKKir703XUcBeaBpbnFz3 ETtX8Dnv//CnXs9fpJnkLku6o7/ODBsyNAcymBNp3I5y80zrxdyKqT6853qwRujknu HRL18GIFAsZdNua7nARtRPQiPHaXK/Z/II2I+NYz1FA1zWaAZuF1rjRtv1jobLfPjx uIPYUsqxGbPbvMEtrQVG/F9Bc9PezRuuDpKSUMgO48Cgtht+onxBHLmcFYSYAY/Y1n YTHkk6DfBUFXA== X-Mailer: emacs 30.2 (via feedmail 11-beta-1 I) From: Aneesh Kumar K.V To: Will Deacon 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 , Marc Zyngier , Steven Price , Suzuki K Poulose , Catalin Marinas , Jiri Pirko , Jason Gunthorpe , Mostafa Saleh , 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 In-Reply-To: References: <20260717180442.110954-18-aneesh.kumar@kernel.org> <20260807092612.2202005-1-aneesh.kumar@kernel.org> Date: Fri, 07 Aug 2026 23:29:54 +0530 Message-ID: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain Will Deacon writes: > On Fri, Aug 07, 2026 at 06:33:14PM +0530, Aneesh Kumar K.V wrote: >> Will Deacon writes: >> >> > On Fri, Aug 07, 2026 at 02:56:12PM +0530, Aneesh Kumar K.V (Arm) wrote: >> >> Realm guests and protected KVM guests may require swiotlb for device DMA >> >> even when all system RAM is addressable by the DMA zones. >> >> >> >> Do not reduce the SWIOTLB buffer to the kmalloc-bouncing size for these >> >> guests, as the reduced number of slots can be exhausted during CCA guest >> >> operation. >> >> >> >> Fixes: 30c5e45ee2c5 ("dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED") >> >> Signed-off-by: Aneesh Kumar K.V (Arm) >> >> --- >> >> arch/arm64/mm/init.c | 3 ++- >> >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> >> >> diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c >> >> index e308a7cabd12..24caaf10e755 100644 >> >> --- a/arch/arm64/mm/init.c >> >> +++ b/arch/arm64/mm/init.c >> >> @@ -339,7 +339,8 @@ void __init arch_mm_preinit(void) >> >> { >> >> unsigned int flags = SWIOTLB_VERBOSE; >> >> >> >> - if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> >> + if (!cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && >> >> + max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> > >> > Protected guests under pKVM rely on restricted DMA, so won't this just >> > waste memory for them? >> > >> >> commit 30c5e45ee2c5 ("dma-direct: make dma_direct_map_phys() honor DMA_ATTR_CC_SHARED"), >> which this patch fixes, has >> >> --- a/arch/arm64/mm/init.c >> +++ b/arch/arm64/mm/init.c >> @@ -339,9 +339,7 @@ void __init arch_mm_preinit(void) >> { >> unsigned int flags = SWIOTLB_VERBOSE; >> >> - if (is_realm_world() || is_protected_kvm_guest()) { >> - flags |= SWIOTLB_FORCE; >> - } else if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> + if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> /* >> * If no bouncing needed for ZONE_DMA, reduce the swiotlb >> * buffer for kmalloc() bouncing to 1MB per 1GB of RAM. >> >> The is_protected_kvm_guest() part was added by the >> commit e62decaf98e7 ("arm64/coco: Add pKVM as a CC platform") >> >> @@ -337,7 +339,7 @@ void __init arch_mm_preinit(void) >> { >> unsigned int flags = SWIOTLB_VERBOSE; >> >> - if (is_realm_world()) { >> + if (is_realm_world() || is_protected_kvm_guest()) { >> flags |= SWIOTLB_FORCE; >> } else if (max_pfn <= PFN_DOWN(arm64_dma_phys_limit)) { >> /* >> @@ -412,6 +414,17 @@ void dump_mem_limit(void) >> } >> } >> >> I was under the impression that there is a possibility of using swiotlb >> instead of restricted-dma-pool with pKVM. > > Yes, that patch enables swiotlb as a possibility for protected guests > but with your patch we avoid shrinking the swiotlb buffer even when > restricted dma pools are being used and that's a waste of memory. > The patch restores the behavior to what it was before that commit. That is, for both CCA and pKVM, we don't do the max_pfn check, and hence the swiotlb is never resized. > >> If that is not the case, then we could change: >> >> !cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && >> >> to >> >> !is_realm_world() && > > Perhaps, or you could just pass the swiotlb= option if the defaults don't > work for you. Can you give more details about the slots exhaustion you're > seeing under CCA? That is definitely possible. However, on arm64, the current code further reduces the default swiotlb size when max_pfn < arm64_dma_phys_limit. That reduction is only correct when swiotlb is being used as a bounce buffer because of arm64_dma_phys_limit. With CCA, that is not the case. We use swiotlb to bounce DMA for untrusted devices. Therefore, reducing the default swiotlb size based on max_pfn is not correct for CCA configurations. This change in behavior is also a regression for CCA. CCA configurations that booted successfully before now gives the error below. [ 39.848004] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.002654] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.165933] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 256 (slots), used 159 (slots) [ 40.325465] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 512 (slots), used 415 (slots) [ 40.485932] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 204800 bytes), total 768 (slots), used 671 (slots) [ 42.485337] EXT4-fs (vda): re-mounted 6870157e-2795-475e-aad6-3a05725fb7cf. [ 43.799800] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1280 (slots), used 1153 (slots) [ 44.016682] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1280 (slots), used 1153 (slots) [ 44.226566] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1536 (slots), used 1409 (slots) [ 44.437194] virtio-pci 0000:00:04.0: swiotlb buffer is full (sz: 262144 bytes), total 1792 (slots), used 1665 (slots) -aneesh