On 30.08.2018 16:18, David Hildenbrand wrote:
> 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)
Yes, currently highly unlikely.
It's not the GMAP that bothers me here, it's the bitfield within the
context. If we are very unlucky, we could loose a write to it.
The locking is a bit interesting. The context has a spinlock, but the
bitfield is protected by mm sem because for skey enablement and SIE
enablement we need to take it for mm operations anyhow.
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