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. > 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. -- 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
