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. Stefan
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