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 DC351471D0B; Wed, 7 Oct 2026 12:15:39 +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=1791375352; cv=none; b=J+7BC44gAc6m8X3I8zUnrnQ5qtP2TggZqYhMqPf2wE/zvYcpUeW4vMxBokkPHnStwl2k3+70Iy7MXe55mmSQB0S/oedwMenGbYYacThK4L3x8Q678MOo1hsJN1h+w+iAAr1IBRXB5HqCtliv3jD5OmCwUUjruFp5ByXeKHHVf3o= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791375352; c=relaxed/simple; bh=TZ42zsc4iA70+OZO19otIceqXRdH1GK8Esf9LXlH/as=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=i5Sydpb6YaONg5ez/ubYPp/Y0bd4TsELcqWZvYPDjq7zUW/4Bm9Wbbs/ByZMF3nuptOW85e7d5EONQrNdfsD9dEXyE9FQrsPS9qRg5CAo6y1SIy+z8I7KsUSTYLzTncr50yfApJwQaG0BP52SpcsUMvGLyPT08eLCJwaVL3g+rA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Bfl8aja+; 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="Bfl8aja+" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 81E961F0089B; Wed, 7 Oct 2026 12:15:22 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791375339; bh=THCKMx4ZCnm8mOHpovi4eI/Rqy36KGJtBfYIVu2eZq8=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=Bfl8aja+VSRTna257B4Gm2/By+1W2Nv4TQy25TzrHnAFX8A1jdDrmXdZ3WzgIyzp8 jBTkfVwJtKCnodEsgOCFyt/BfpUeE6IZHCbxRI9zna/h0rF0LbfWDm9i4r2aHrawGI IGfikya6X0AbG64Zsqz5emGEUelpLSovqtyGoPLiIVKkvcZkkrJMeSGpIXUcZvpiUG mO1yAEOlwfQ1bj2VR8aFJGvRCIWlqYxiNFzVrp24UHu7ugy8E3RDe8t8FmvS7Zeixg 2pF40rwJ7JSNIgBhu+4qSL0O0WS2aBAOGN6PLwSVDSRzDWBbTgQlO/Dl8ONXKChE5p yOivzKvQiEetQ== X-Mailer: emacs 31.1 (via feedmail 11-beta-1 I) From: Aneesh Kumar K.V To: Will Deacon , Catalin Marinas Cc: iommu@lists.linux.dev, linux-kernel@vger.kernel.org, Robin Murphy , Marek Szyprowski , Jonathan Corbet , Shuah Khan , Randy Dunlap , Mark Rutland , Marc Zyngier , Steven Price , Suzuki K Poulose , Jiri Pirko , Jason Gunthorpe , Mostafa Saleh , Petr Tesarik , Alexey Kardashevskiy , Dan Williams , Xu Yilun , Madhavan Srinivasan , Michael Ellerman , Nicholas Piggin , "Christophe Leroy (CS GROUP)" , "Ritesh Harjani (IBM)" , Shrikanth Hegde , Alexander Gordeev , Gerald Schaefer , Heiko Carstens , Vasily Gorbik , Christian Borntraeger , Sven Schnelle , Stefano Stabellini , Russell King , Huacai Chen , WANG Xuerui , Thomas Bogendoerfer , Jiaxun Yang , Paul Walmsley , Palmer Dabbelt , Albert Ou , Alexandre Ghiti , Andy Lutomirski , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , "H. Peter Anvin" , linux-arm-kernel@lists.infradead.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org, linux-s390@vger.kernel.org, x86@kernel.org Subject: Re: [PATCH v6 6/8] dma: swiotlb: Centralize memory-encryption pool sizing In-Reply-To: References: <20260924060756.1325156-1-aneesh.kumar@kernel.org> <20260924060756.1325156-7-aneesh.kumar@kernel.org> Date: Wed, 07 Oct 2026 17:45:19 +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 Wed, Oct 07, 2026 at 11:04:05AM +0100, Catalin Marinas wrote: >> On Tue, Oct 06, 2026 at 10:49:17PM +0100, Will Deacon wrote: >> > On Thu, Sep 24, 2026 at 11:37:54AM +0530, Aneesh Kumar K.V (Arm) wrote: >> > > @@ -496,7 +516,8 @@ swiotlb_select_pool_policy(unsigned int flags) >> > > if (swiotlb_force_disable) >> > > return SWIOTLB_POOL_NONE; >> > > >> > > - if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT)) >> > > + if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && >> > > + !restricted_dma_pool_present) >> > > return SWIOTLB_POOL_CC_GUEST; >> > >> > I think this check on the restricted DMA pool is too general -- the pool >> > could be tied to a specific DMA-capable peripheral and so treating its >> > presence as a global property isn't right. >> >> I agree it's a hack but that was the simplest way to avoid the pVMs >> getting a bounce buffer after this patch. More than happy to leave it >> out and reduce the buffer on cmdline or we come up with some better >> heuristics. > > Hrm, that does mean that reverting just this part will regress pVMs > because they'll suddenly be allocating a tonne more memory for an > entirely unused swiotlb buffer. So I think I'd prefer to drop the entire > series until this has been worked out properly. > >> Another option could be the arch code passing another flag that it >> doesn't want an encrypted pool (e.g. when running in a pKVM guest) but I >> don't particularly this either. The arch code doesn't know whether >> there's an alternative pool. > > At that point, the default size may as well be driven by the > drivers/virt/coco driver. > >> That said, such heuristics should have been a separate patch to make it >> easier to review/drop. > > I think the only right way to get a semi-accurate heuristic is to take > into account the set of dma-capable devices that will use the swiotlb > pool, but that's fiddly and should probably be tackled as a separate > series. Maybe a simpler hack in that direction would be to take the > SWIOTLB_POOL_CC_GUEST if _any_ device is going to use swiotlb? You'll > run into the usual problem of not being able to tell if a device is > DMA-capable or not, but you could probably look for a global restricted > DMA pool and, if that doesn't exist, check for per-device restricted pools > on dma-coherent devices (since restricted DMA isn't supported by ACPI) as > a reasonable approximation. So, something like this? if (cc_platform_has(CC_ATTR_GUEST_MEM_ENCRYPT) && swiotlb_cc_guest_needs_default_pool()) return SWIOTLB_POOL_CC_GUEST; where: static bool swiotlb_cc_guest_needs_default_pool(void) { for_each_of_allnodes(np) { if (swiotlb_of_dma_needs_default_pool(np)) return true; } return false; } I didn't quite follow the ACPI-related feedback. IIUC, on ACPI systems we will end up using SWIOTLB_POOL_CC_GUEST? -aneesh