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

[Martin Schwidefsky <schwidefsky@de.ibm.com>]

On Fri, 2 Jun 2017 09:25:54 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> 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@de.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)

Exactly-

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.