On Thu, 24 Jan 2019, Peng Fan wrote: > Hi stefano, > > > -----Original Message----- > > From: Stefano Stabellini [mailto:sstabell...@kernel.org] > > Sent: 2019年1月24日 7:44 > > To: h...@infradead.org > > Cc: Stefano Stabellini <sstabell...@kernel.org>; Peng Fan > > <peng....@nxp.com>; m...@redhat.com; jasow...@redhat.com; > > xen-de...@lists.xenproject.org; linux-remotep...@vger.kernel.org; > > linux-kernel@vger.kernel.org; virtualizat...@lists.linux-foundation.org; > > l...@kernel.org; jgr...@suse.com; boris.ostrov...@oracle.com; > > bjorn.anders...@linaro.org; jli...@xilinx.com > > Subject: Re: [Xen-devel] [RFC] virtio_ring: check dma_mem for xen_domain > > > > On Wed, 23 Jan 2019, h...@infradead.org wrote: > > > On Wed, Jan 23, 2019 at 01:04:33PM -0800, Stefano Stabellini wrote: > > > > If vring_use_dma_api is actually supposed to return true when > > > > dma_dev->dma_mem is set, then both Peng's patch and the patch I > > > > wrote are not fixing the real issue here. > > > > > > > > I don't know enough about remoteproc to know where the problem > > > > actually lies though. > > > > > > The problem is the following: > > > > > > Devices can declare a specific memory region that they want to use > > > when the driver calls dma_alloc_coherent for the device, this is done > > > using the shared-dma-pool DT attribute, which comes in two variants > > > that would be a little to much to explain here. > > > > > > remoteproc makes use of that because apparently the device can only > > > communicate using that region. But it then feeds back memory obtained > > > with dma_alloc_coherent into the virtio code. For that it calls > > > vmalloc_to_page on the dma_alloc_coherent, which is a huge no-go for > > > the ĐMA API and only worked accidentally on a few platform, and > > > apparently arm64 just changed a few internals that made it stop > > > working for remoteproc. > > > > > > The right answer is to not use the DMA API to allocate memory from a > > > device-speficic region, but to tie the driver directly into the DT > > > reserved memory API in a way that allows it to easilt obtain a struct > > > device for it. > > > > If I understand correctly, Peng should be able to reproduce the problem on > > native Linux without any Xen involvement simply by forcing > > vring_use_dma_api to return true. Peng, can you confirm? > > It is another issue without xen involvement, > There is an thread talking this: https://patchwork.kernel.org/patch/10742923/ > > Without xen, vring_use_dma_api will return false. > With xen, if vring_use_dma_api returns true, it will dma_map_xx and trigger > dump.
It is true that for Xen on ARM DomUs it is not necessary today to return true from vring_use_dma_api. However, returning true from vring_use_dma_api should not break Linux. When the rpmesg issue is fixed, this problem should also go away without any need for additional changes on the xen side I think.
