On 19.09.2017 14:48, Thomas Huth wrote: > On 19.09.2017 14:38, David Hildenbrand wrote: >> On 18.09.2017 09:43, Christian Borntraeger wrote: >>> >>> >>> On 09/15/2017 04:36 PM, Thomas Huth wrote: >>>> On 29.03.2017 16:25, Christian Borntraeger wrote: >>>>> On 03/29/2017 04:21 PM, Thomas Huth wrote: >>>>>> On 24.03.2017 10:39, Christian Borntraeger wrote: >>>>>>> On 03/24/2017 10:26 AM, Thomas Huth wrote: >>>>>>>> When running QEMU with KVM under z/VM, the memory for the guest >>>>>>>> is allocated via legacy_s390_alloc() since the KVM_CAP_S390_COW >>>>>>>> extension is not supported on z/VM. legacy_s390_alloc() then uses >>>>>>>> mmap(... PROT_EXEC ...) for the guest memory - but this does not >>>>>>>> work when running with SELinux enabled, mmap() fails and QEMU aborts >>>>>>>> with the following error message: >>>>>>>> >>>>>>>> cannot set up guest memory 's390.ram': Permission denied >>>>>>>> >>>>>>>> Looking at the other allocator function qemu_anon_ram_alloc(), it >>>>>>>> seems like PROT_EXEC is normally not needed for allocating the >>>>>>>> guest RAM, and indeed, the guest also starts successfully under >>>>>>>> z/VM when we remove the PROT_EXEC from the legacy_s390_alloc() >>>>>>>> function. So let's get rid of that flag here to be able to run >>>>>>>> with SELinux under z/VM, too. >>>>>>> >>>>>>> Older z/VM versions do not provide the enhanced suppression on >>>>>>> protection >>>>>>> facility, which would result in guest failures as soon as the kernel >>>>>>> starts dirty pages tracking by write protecting the pages via the page >>>>>>> table. Some kernel release back (last time I checked) the PROT_EXEC was >>>>>>> necessary to prevent the dirty pages tracking from taking place. So this >>>>>>> patch would break KVM in that case. >>>>>>> >>>>>>> Newer z/VMs (e.g. 6.3) do provide ESOP. SO the question is, >>>>>>> why is KVM_CAP_S390_COW not set? >>>>>> >>>>>> I now had another look at this, and seems like the ESOP bit is indeed >>>>>> not set in S390_lowcore.machine_flags here. According to /proc/sysinfo, >>>>>> z/VM is version 6.1.0 here, so I guess that's just too old for ESOP? >>>>> >>>>> Yes, this was introduced with z/VM 6.3 >>>> >>>> FWIW, the last version without ESOP, z/VM 6.2, is now end of life, >>>> according to: http://www.vm.ibm.com/techinfo/lpmigr/vmleos.html >>>> ... so I guess we could remove the legacy_s390_alloc() function now? >>> >>> >>> I recently learned that you can buy some extended z/VM support not sure how >>> long this will be available. In addition, ESOP was added with z10, so >>> if we still care about z9 and older then this would break things on >>> very very old boxes. >> >> I wonder if that is really relevant anymore. >> >> Existing user on such machines (I doubt there are many) can simply stick >> to QEMU <= 2.10. Or do we actually expect people with such old >> environments to use latest and grates QEMU versions? >> >> We could add an error message an error out. > > Well, as long as the code does not cause any trouble for us, and as long > as there still might be possible users, there is also no real urge to > remove it, is there? I originally thought that all affected systems > would now be EOL, but as Christian pointed out, the z9 BC is not EOL > yet, so I'd say we should at least wait for that point in time before > removing it (I haven't found any public information about extended z/VM > support though, so no clue whether we should really take that into account). > > Thomas >
It's the last remaining alloc hack we have in QEMU :) That's why I am asking the question. -- Thanks, David
