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?
Thomas
