On Thu, 3 Dec 2020 10:43:22 -0500 Peter Xu <pet...@redhat.com> wrote:
> On Thu, Dec 03, 2020 at 11:20:02AM +0000, Stefan Hajnoczi wrote: > > 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. > > Sounds right. One more thing to mention is that we may need to avoid tearing > down the whole old DMA region when resizing the PROT_READ|PROT_WRITE region > into e.g. a bigger one to cover some of the previusly PROT_NONE part, as long > as if the before-resizing region is still possible to be accessed from any > hardware. It smells like something David is working with virtio-mem, not sure > whether there's any common infrastructure that could be shared. Yes, very similar to his RamDiscardMgr which works on a granularity basis. vfio maps in granularity chunks and unmaps can span multiple chunks. This usage might be similar enough to use as-is. Thanks, Alex
