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.
