On 08/14/2012 11:25 PM, Marcelo Tosatti wrote: > On Tue, Aug 14, 2012 at 10:58:07AM +0800, Xiao Guangrong wrote: >> On 08/14/2012 01:39 AM, Marcelo Tosatti wrote: >>> On Sat, Aug 11, 2012 at 11:36:20AM +0800, Xiao Guangrong wrote: >>>> On 08/11/2012 02:14 AM, Marcelo Tosatti wrote: >>>>> On Tue, Aug 07, 2012 at 05:47:15PM +0800, Xiao Guangrong wrote: >>>>>> Changelog: >>>>>> - introduce KVM_PFN_ERR_RO_FAULT instead of dummy page >>>>>> - introduce KVM_HVA_ERR_BAD and optimize error hva indicators >>>>>> >>>>>> The test case can be found at: >>>>>> http://lkml.indiana.edu/hypermail/linux/kernel/1207.2/00819/migrate-perf.tar.bz2 >>>>>> >>>>>> In current code, if we map a readonly memory space from host to guest >>>>>> and the page is not currently mapped in the host, we will get a fault-pfn >>>>>> and async is not allowed, then the vm will crash. >>>>>> >>>>>> As Avi's suggestion, We introduce readonly memory region to map ROM/ROMD >>>>>> to the guest, read access is happy for readonly memslot, write access on >>>>>> readonly memslot will cause KVM_EXIT_MMIO exit. >>>>> >>>>> Memory slots whose QEMU mapping is write protected is supported >>>>> today, as long as there are no write faults. >>>>> >>>>> What prevents the use of mmap(!MAP_WRITE) to handle read-only memslots >>>>> again? >>>>> >>>> >>>> It is happy to map !write host memory space to the readonly memslot, >>>> and they can coexist as well. >>>> >>>> readonly memslot checks the write-permission by seeing slot->flags and >>>> !write memory checks the write-permission in hva_to_pfn() function >>>> which checks vma->flags. It is no conflict. >>> >>> Yes, there is no conflict. The point is, if you can use the >>> mmap(PROT_READ) interface (supporting read faults on read-only slots) >>> for this behavior, what is the advantage of a new memslot flag? >>> >> >> You can get the discussion at: >> https://lkml.org/lkml/2012/5/22/228 >> >>> I'm not saying mmap(PROT_READ) is the best interface, i am just asking >>> why it is not. >> >> My fault. :( >> >>> >>>>> The initial objective was to fix a vm crash, can you explain that >>>>> initial problem? >>>>> >>>> >>>> The issue was trigged by this code: >>>> >>>> } else { >>>> if (async && (vma->vm_flags & VM_WRITE)) >>>> *async = true; >>>> pfn = KVM_PFN_ERR_FAULT; >>>> } >>>> >>>> If the host memory region is readonly (!vma->vm_flags & VM_WRITE) and >>>> its physical page is swapped out (or the file data does not be read in), >>>> get_user_page_nowait will fail, above code reject to set async, >>>> then we will get a fault pfn and async=false. >>>> >>>> I guess this issue also exists in "QEMU write protected mapping" as >>>> you mentioned above. >>> >>> Yes, it does. As far as i understand, what that check does from a high >>> level pov is: >>> >>> - Did get_user_pages_nowait() fail due to a swapped out page (in which >>> case we should try to swappin the page asynchronously), or due to >>> another reason (for which case an error should be returned). >>> >>> Using vma->vm_flags VM_WRITE for that is trying to guess why >>> get_user_pages_nowait() failed, because it (gup_nowait return values) >>> does not provide sufficient information by itself. >>> >> >> That is exactly what i did in the first version. :) >> >> You can see it and the reason why it switched to the new way (readonly memslot) >> in the above website (the first message in thread). > > Userspace can create multiple mappings for the same memory region, for > example via shared memory (shm_open), and have different protections for > the two (or more) regions. I had old patch doing this, its attached. > In this way, if guest try to write a readonly gfn, the vm will be crashed since it will return FAULT_PFN on the page-fault path. VMM can not detect this kind of fault, we have these problems: - even if guest try to write ROM on a PCI device, the guest will die, but we'd ignore this write, it looks more like the real machine. - can not implement ROMD beacuse write to a ROMD is MMIO access Yes, we can rework get_user_page_nowait and get_user_pages_fast, let them tell us the fault reason, but it is more complex i think. >>> Can't that be fixed separately? >>> >>> Another issue which is also present with the mmap(PROT_READ) scheme is >>> interaction with reexecute_instruction. That is, unless i am mistaken, >>> reexecute_instruction can succeed (return true) on a region that is >>> write protected. This breaks the "write faults on read-only slots exit >>> to userspace via EXIT_MMIO" behaviour. >> >> Sorry, Why? After re-entry to the guest, it can not generate a correct MMIO? > > reexecute_instruction validates presence of GPA by looking at registered > memslots. But if the access is a write, and userspace memory map is > read-only, reexecute_instruction should exit via MMIO. > > That is, reexecute_instruction must validate GPA using registered > memslots AND additionaly userspace map permission, not only registered > memslot. > What will happen if we always retry a unhandleable instruction which try to write readonly memory? It will goto a endless loop (write-fault -> emulation fail -> write-fault...)? Right? I do not think exit via MMIO is a good idea because the instructions can not be emulated, after the userspace finished the MMIO, the emulation will fail again. I think we can simply exit via KVM_EXIT_INTERNAL_ERROR for all the access on readonly memory because: - it is fine for the read access since the read fault is always fixed on page-fault path, it does not go to x86_emulate_instruction() - for the write access, we can not emulate it. It is not bad since it only happen on the instructions kvm unsupported. Your idea? -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
