On 31.08.2018 10:48, Christian Borntraeger wrote:
>
>
> On 08/31/2018 09:52 AM, David Hildenbrand wrote:
>> On 31.08.2018 08:20, Janosch Frank wrote:
>>> 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.
>>
>> Right, although I think this is also very theoretical. Never the less,
>> this is certainly the right thing to do.
>
> Yes. The compiler might even use the oi instruction (which is on all newer machines
> atomic in terms of setting bits).
>>
>> (I am just wondering if this one is worth backporting, do you see the need?)
>
> This also fixes a patch that was added after 4.18 so we should just add it for 4.19.
>
Yes, backports to stable do not apply, but maybe to something else ;)
--
Thanks,
David / dhildenb
