From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from verein.lst.de ([213.95.11.211]:42203 "EHLO verein.lst.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726019AbfKZSpa (ORCPT ); Tue, 26 Nov 2019 13:45:30 -0500 Date: Tue, 26 Nov 2019 19:45:27 +0100 From: Christoph Hellwig Subject: Re: [PATCH 1/1] virtio_ring: fix return code on DMA mapping fails Message-ID: <20191126184527.GA10481@lst.de> References: <20191114124646.74790-1-pasic@linux.ibm.com> <20191119121022.03aed69a.pasic@linux.ibm.com> <20191119080420-mutt-send-email-mst@kernel.org> <20191122140827.0ead345c.pasic@linux.ibm.com> <1ec7c229-6c4f-9351-efda-ed2df20f95f6@amd.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1ec7c229-6c4f-9351-efda-ed2df20f95f6@amd.com> Sender: linux-s390-owner@vger.kernel.org List-ID: To: Tom Lendacky Cc: Halil Pasic , "Michael S. Tsirkin" , Jason Wang , virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, Cornelia Huck , linux-s390@vger.kernel.org, Michael Mueller , Christian Borntraeger , Janosch Frank , Christoph Hellwig , Ram Pai , Thiago Jung Bauermann , Andy Lutomirski , Brijesh Singh , "Kalra, Ashish" On Sat, Nov 23, 2019 at 09:39:08AM -0600, Tom Lendacky wrote: > Ideally, having a pool of shared pages for DMA, outside of standard > SWIOTLB, might be a good thing. On x86, SWIOTLB really seems geared > towards devices that don't support 64-bit DMA. If a device supports 64-bit > DMA then it can use shared pages that reside anywhere to perform the DMA > and bounce buffering. I wonder if the SWIOTLB support can be enhanced to > support something like this, using today's low SWIOTLB buffers if the DMA > mask necessitates it, otherwise using a dynamically sized pool of shared > pages that can live anywhere. I think that can be done relatively easily. I've actually been thinking of multiple pool support for a whіle to replace the bounce buffering in the block layer for ISA devices (24-bit addressing). I've also been looking into a dma_alloc_pages interface to help people just allocate pages that are always dma addressable, but don't need a coherent allocation. My last version I shared is here: http://git.infradead.org/users/hch/misc.git/shortlog/refs/heads/dma_alloc_pages But it turns out this still doesn't work with SEV as we'll always bounce. And I've been kinda lost on figuring out a way how to allocate unencrypted pages that we we can feed into the normal dma_map_page & co interfaces due to the magic encryption bit in the address. I guess we could have a fallback path in the mapping path and just unconditionally clear that bit in the dma_to_phys path.