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 > <dgilbert@xxxxxxxxxx> wrote: > > > > * Dan Williams (dan.j.williams@xxxxxxxxx) wrote: > > > On Mon, Dec 10, 2018 at 9:22 AM Vivek Goyal <vgoyal@xxxxxxxxxx> wrote: > > > > > > > > From: Stefan Hajnoczi <stefanha@xxxxxxxxxx> > > > > > > > > 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.
