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? I'm not saying mmap(PROT_READ) is the best interface, i am just asking why it is not. > > 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. 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. -- 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
