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Fri, 7 Aug 2026 13:18:35 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786108720; bh=wsfDDPcsG5MUSLOHvWCFv40tfsXBmUj7P+V3Bqk82YU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=cCNbiOkjKWAUqmhRd4f35UKCjy7UEhkqpWWFeU9w7Kfiy5DgWT3dZw1OQFG8VAzwR XZNBYUU8FhCvDz09mx7t85bJh2lGZS5wWKXfdEHcjdTmqrCKuvbXD4PHj259GvbZZY At6gmah61p+afUyBaMnPfpe+im9eRwJlcM1uOVFcvRENntNyyd8OspaulZRPI21O0+ RQ+DtFZQKiusnOPhpgTJWrPlMW7ENlDI/iDnaojBH384ZJnryZxsxnSTIXfU3vj3ks R4W0nC24in4QB64L5eYdsvT8dTMBFtjRhb+mL0yI6d/AOEHHOKCqEgpBrbJoOp2NCH LWSkcbZumcQJg== Date: Fri, 7 Aug 2026 14:18:31 +0100 From: Will Deacon 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 , 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 Message-ID: References: <20260717180442.110954-18-aneesh.kumar@kernel.org> <20260807092612.2202005-1-aneesh.kumar@kernel.org> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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. > 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? Will