Re: [PATCH RFC 0/2] KVM: s390: avoid having to enable vm.alloc_pgste

[Christian Borntraeger]

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.