On 30.01.20 11:56, Thomas Huth wrote:
> On 30/01/2020 11.39, Cornelia Huck wrote:
>> On Thu, 30 Jan 2020 10:49:35 +0100
>> David Hildenbrand <david@xxxxxxxxxx> wrote:
>>
>>> On 30.01.20 09:55, Christian Borntraeger wrote:
>>>> The initial CPU reset currently clobbers the userspace fpc. This was an
>>>> oversight during a fixup for the lazy fpu reloading rework. The reset
>>>> calls are only done from userspace ioctls. No CPU context is loaded, so
>>>> we can (and must) act directly on the sync regs, not on the thread
>>>> context. Otherwise the fpu restore call will restore the zeroes fpc to
>>>> userspace.
>>>>
>>>> Cc: stable@xxxxxxxxxx
>>>> Fixes: 9abc2a08a7d6 ("KVM: s390: fix memory overwrites when vx is disabled")
>>>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>>>> ---
>>>> arch/s390/kvm/kvm-s390.c | 3 +--
>>>> 1 file changed, 1 insertion(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index c059b86..eb789cd 100644
>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>> @@ -2824,8 +2824,7 @@ static void kvm_s390_vcpu_initial_reset(struct kvm_vcpu *vcpu)
>>>> vcpu->arch.sie_block->gcr[14] = CR14_UNUSED_32 |
>>>> CR14_UNUSED_33 |
>>>> CR14_EXTERNAL_DAMAGE_SUBMASK;
>>>> - /* make sure the new fpc will be lazily loaded */
>>>> - save_fpu_regs();
>>>> + vcpu->run->s.regs.fpc = 0;
>>>> current->thread.fpu.fpc = 0;
>>>> vcpu->arch.sie_block->gbea = 1;
>>>> vcpu->arch.sie_block->pp = 0;
>>>>
>>>
>>> kvm_arch_vcpu_ioctl() does a vcpu_load(vcpu), followed by the call to
>>> kvm_arch_vcpu_ioctl_initial_reset(), followed by a vcpu_put().
>>>
>>> What am I missing?
>>
>> I have been staring at this patch for some time now, and I fear I'm
>> missing something as well. Can we please get more explanation?
>
> Could we please get a test for this issue in the kvm selftests, too?
> I.e. host sets a value in its FPC, then calls the INITIAL_RESET ioctl
> and then checks that the value in its FPC is still there?
Yes, that will come as a later addon patch. (But I am still going to apply
this series soon and do not wait for that. I have a private hack in qemu that
does this checking, but this test code is too ugly to see the world).
