On Thu, Aug 16, 2012 at 01:49:11PM +0800, Xiao Guangrong wrote: > 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 think so... thats what would happen on real hardware. > 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? Either KVM_EXIT_INTERNAL_ERROR or leave the guest looping. -- 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
