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

[Martin Schwidefsky]

On Fri, 2 Jun 2017 09:13:03 +0200
Christian Borntraeger <borntraeger@xxxxxxxxxx> 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.

-- 
blue skies,
   Martin.

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