From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com ([209.132.183.28]:34678 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726401AbeLMUkz (ORCPT ); Thu, 13 Dec 2018 15:40:55 -0500 Date: Thu, 13 Dec 2018 15:40:52 -0500 From: Vivek Goyal To: Dan Williams Cc: "Dr. David Alan Gilbert" , linux-fsdevel , Linux Kernel Mailing List , KVM list , Miklos Szeredi , Stefan Hajnoczi , sweil@redhat.com, Steven Whitehouse Subject: Re: [PATCH 15/52] fuse: map virtio_fs DAX window BAR Message-ID: <20181213204052.GE4384@redhat.com> References: <20181210171318.16998-1-vgoyal@redhat.com> <20181210171318.16998-16-vgoyal@redhat.com> <20181213200936.GU2313@work-vm> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Thu, Dec 13, 2018 at 12:15:51PM -0800, Dan Williams wrote: > On Thu, Dec 13, 2018 at 12:09 PM Dr. David Alan Gilbert > wrote: > > > > * Dan Williams (dan.j.williams@intel.com) wrote: > > > On Mon, Dec 10, 2018 at 9:22 AM Vivek Goyal wrote: > > > > > > > > From: Stefan Hajnoczi > > > > > > > > Experimental QEMU code introduces an MMIO BAR for mapping portions of > > > > files in the virtio-fs device. Map this BAR so that FUSE DAX can access > > > > file contents from the host page cache. > > > > > > FUSE DAX sounds terrifying, can you explain a bit more about what this is? > > > > We've got a guest running in QEMU, it sees an emulated PCI device; > > that runs a FUSE protocol over virtio on that PCI device, but also has > > a trick where via commands sent over the virtio queue associated with that device, > > (fragments of) host files get mmap'd into the qemu virtual memory that corresponds > > to the kvm slot exposed to the guest for that bar. > > > > The guest sees those chunks in that BAR, and thus you can read/write > > to the host file by directly writing into that BAR. > > Ok so it's all software emulated and there won't be hardware DMA > initiated by the guest to that address? That's my understanding. > I.e. if the host file gets > truncated / hole-punched the guest would just cause a refault and the > filesystem could fill in the block, Right > or the guest is expected to die if > the fault to the truncated file range results in SIGBUS. Are you referring to the case where a file page is mapped in qemu and another guest/process trucates that page and when qemu tries to access it it will get SIGBUS. Have not tried it, will give it a try. Not sure what happens when QEMU receives SIGBUS. Having said that, this is not different from the case of one process mapping a file and another process truncating the file and first process getting SIGBUS, right? Thanks Vivek > > > > > The DAX window is accessed by the fs/dax.c infrastructure and must have > > > > struct pages (at least on x86). Use devm_memremap_pages() to map the > > > > DAX window PCI BAR and allocate struct page. > > > > > > PCI BAR space is not cache coherent, > > > > Note that no real PCI infrastructure is involved - this is all emulated > > devices, backed by mmap'd files on the host qemu process. > > Ok, terror level decreased.