linux-kernel - Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc

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Date:   Fri, 2 Jun 2017 09:25:54 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Martin Schwidefsky <schwidefsky@...ibm.com>
Cc:     Heiko Carstens <heiko.carstens@...ibm.com>,
        David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Huth <thuth@...hat.com>
Subject: Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc_pgste

On 06/02/2017 09:18 AM, Christian Borntraeger wrote:
> On 06/02/2017 09:16 AM, Martin Schwidefsky wrote:
>> On Fri, 2 Jun 2017 09:13:03 +0200
>> Christian Borntraeger <borntraeger@...ibm.com> wrote:
>>
>>> On 06/02/2017 09:02 AM, Heiko Carstens wrote:
>>>> On Thu, Jun 01, 2017 at 12:46:51PM +0200, Martin Schwidefsky wrote:  
>>>>>> Unfortunately, converting all page tables to 4k pgste page tables is
>>>>>> not possible without provoking various race conditions.  
>>>>>
>>>>> That is one approach we tried and was found to be buggy. The point is that
>>>>> you are not allowed to reallocate a page table while a VMA exists that is
>>>>> in the address range of that page table.
>>>>>
>>>>> Another approach we tried is to use an ELF flag on the qemu executable.
>>>>> That does not work either because fs/exec.c allocates and populates the
>>>>> new mm struct for the argument pages before fs/binfmt_elf.c comes into
>>>>> play.  
>>>>
>>>> How about if you would fail the system call within arch_check_elf() if you
>>>> detect that the binary requires pgstes (as indicated by elf flags) and then
>>>> restart the system call?
>>>>
>>>> That is: arch_check_elf() e.g. would set a thread flag that future mm's
>>>> should be allocated with pgstes. Then do_execve() would cleanup everything
>>>> and return to entry.S. Upon return to userspace we detect this condition
>>>> and simply restart the system call, similar to signals vs -ERESTARTSYS.
>>>>
>>>> That would make do_execve() cleanup everything and upon reentering it would
>>>> allocate an mm with the pgste flag set.
>>>>
>>>> Maybe this is a bit over-simplified, but might work.
>>>>
>>>> At least I also don't like the next "hack", that is specifically designed
>>>> to only work with how QEMU is currently implemented. It might break with
>>>> future QEMU changes or the next user space implementation that drives the
>>>> kvm interface, but is doing everything differently.
>>>> Let's look for a "clean" solution that will always work. We had too many
>>>> hacks for this problem and *all* of them were broken.  
>>>
>>>
>>> The more I think about it, dropping 2k page tables and always allocate a full
>>> page would simplify pgalloc. As far I can see this would also get rid of
>>> the &mm->context.pgtable_lock.
>>
>> And it would waste twice the amount of memory for page tables. NAK.
> 
> Yes and we spend the same amount of memory TODAY, because every distro on the
> planet that uses KVM has sysctl.allocate_pgste set.

Maybe todays approach might be still the best. (if qemu is installed, its all
4k, if not its all 2k)