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Re: [Qemu-devel] [PATCH] target/s390x/kvm: Fix problem when running with

From: David Hildenbrand
Subject: Re: [Qemu-devel] [PATCH] target/s390x/kvm: Fix problem when running with SELinux under z/VM
Date: Tue, 19 Sep 2017 15:03:56 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

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.




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