On Wed, Dec 02, 2020 at 10:45:11AM -0500, Peter Xu wrote: > On Wed, Dec 02, 2020 at 02:33:56PM +0000, Stefan Hajnoczi wrote: > > On Wed, Nov 25, 2020 at 10:57:11AM -0500, Peter Xu wrote: > > > On Wed, Nov 25, 2020 at 01:05:25AM +0000, Justin He wrote: > > > > > I'd appreciate if you could explain why vfio needs to dma map some > > > > > PROT_NONE > > > > > > > > Virtiofs will map a PROT_NONE cache window region firstly, then remap the sub > > > > region of that cache window with read or write permission. I guess this might > > > > be an security concern. Just CC virtiofs expert Stefan to answer it more accurately. > > > > > > Yep. Since my previous sentence was cut off, I'll rephrase: I was thinking > > > whether qemu can do vfio maps only until it remaps the PROT_NONE regions into > > > PROT_READ|PROT_WRITE ones, rather than trying to map dma pages upon PROT_NONE. > > > > Userspace processes sometimes use PROT_NONE to reserve virtual address > > space. That way future mmap(NULL, ...) calls will not accidentally > > allocate an address from the reserved range. > > > > virtio-fs needs to do this because the DAX window mappings change at > > runtime. Initially the entire DAX window is just reserved using > > PROT_NONE. When it's time to mmap a portion of a file into the DAX > > window an mmap(fixed_addr, ...) call will be made. > > Yes I can understand the rational on why the region is reserved. However IMHO > the real question is why such reservation behavior should affect qemu memory > layout, and even further to VFIO mappings. > > Note that PROT_NONE should likely mean that there's no backing page at all in > this case. Since vfio will pin all the pages before mapping the DMAs, it also > means that it's at least inefficient, because when we try to map all the > PROT_NONE pages we'll try to fault in every single page of it, even if they may > not ever be used. > > So I still think this patch is not doing the right thing. Instead we should > somehow teach qemu that the virtiofs memory region should only be the size of > enabled regions (with PROT_READ|PROT_WRITE), rather than the whole reserved > PROT_NONE region. virtio-fs was not implemented with IOMMUs in mind. The idea is just to install a kvm.ko memory region that exposes the DAX window. Perhaps we need to treat the DAX window like an IOMMU? That way the virtio-fs code can send map/unmap notifications and hw/vfio/ can propagate them to the host kernel. Stefan
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