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

From: Thomas Huth
Subject: Re: [Qemu-devel] [PATCH] target/s390x/kvm: Fix problem when running with SELinux under z/VM
Date: Fri, 15 Sep 2017 16:36:45 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0

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?


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