On 11/8/19 12:19 PM, Christoffer Dall wrote: > Hi, > > I had a look at our relatively complicated logic in > kvm_arch_prepare_memory_region(), and was wondering if there was room to > unify some of this handling between architectures. > > (If you haven't seen our implementation, you can find it in > virt/kvm/arm/mmu.c, and it has lovely ASCII art!) > > I then had a look at the x86 code, but that doesn't actually do anything > when creating memory regions, which makes me wonder why the arhitectures > differ in this aspect. > > The reason we added the logic that we have for arm/arm64 is that we > don't really want to take faults for I/O accesses. I'm not actually > sure if this is a corretness thing, or an optimization effort, and the > original commit message doesn't really explain. Ard, you wrote that > code, do you recall the details? > I have a vague recollection of implementing execution from read-only guest memory in order to support execute-in-place from emulated NOR flash in UEFI, and going down a rabbit hole debugging random, seemingly unrelated crashes in the host which turned out to be caused by the zero page getting corrupted because it was mapped read-write in the guest to back uninitialized regions of the NOR flash. That doesn't quite answer your question, though - I think it was just an optimization ... > In any case, what we do is to check for each VMA backing a memslot, we > check if the memslot flags and vma flags are a reasonable match, and we > try to detect I/O mappings by looking for the VM_PFNMAP flag on the VMA > and pre-populate stage 2 page tables (our equivalent of EPT/NPT/...). > However, there are some things which are not clear to me: > > First, what prevents user space from messing around with the VMAs after > kvm_arch_prepare_memory_region() completes? If nothing, then what is > the value of the cheks we perform wrt. to VMAs? > > Second, why would arm/arm64 need special handling for I/O mappings > compared to other architectures, and how is this dealt with for > x86/s390/power/... ? > > > Thanks, > > Christoffer > IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
