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.
