On 30.08.2018 16:15, Janosch Frank wrote:
> We have to do down_write on the mm semaphore to set a bitfield in the
> mm context.
>
> Signed-off-by: Janosch Frank <frankja@xxxxxxxxxxxxx>
> Fixes: a4499382 ("KVM: s390: Add huge page enablement control")
> ---
> arch/s390/include/asm/mmu.h | 8 +++++++-
> arch/s390/kvm/kvm-s390.c | 2 ++
> 2 files changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/arch/s390/include/asm/mmu.h b/arch/s390/include/asm/mmu.h
> index f31a15044c24..a8418e1379eb 100644
> --- a/arch/s390/include/asm/mmu.h
> +++ b/arch/s390/include/asm/mmu.h
> @@ -16,7 +16,13 @@ typedef struct {
> unsigned long asce;
> unsigned long asce_limit;
> unsigned long vdso_base;
> - /* The mmu context allocates 4K page tables. */
> + /*
> + * The following bitfields need a down_write on the mm
> + * semaphore when they are written to. As they are only
> + * written once, they can be read without a lock.
> + *
> + * The mmu context allocates 4K page tables.
> + */
> unsigned int alloc_pgste:1;
> /* The mmu context uses extended page tables. */
> unsigned int has_pgste:1;
> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> index 8c039e12cb24..ac5da6b0b862 100644
> --- a/arch/s390/kvm/kvm-s390.c
> +++ b/arch/s390/kvm/kvm-s390.c
> @@ -695,7 +695,9 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm, struct kvm_enable_cap *cap)
> r = -EINVAL;
> else {
> r = 0;
> + down_write(&kvm->mm->mmap_sem);
> kvm->mm->context.allow_gmap_hpage_1m = 1;
> + up_write(&kvm->mm->mmap_sem);
> /*
> * We might have to create fake 4k page
> * tables. To avoid that the hardware works on
>
This is a theoretical scenario, right?
(GMAP without vcpus will not be populated as far as I remember)
--
Thanks,
David / dhildenb
