Re: [PATCH v4 24/36] KVM: s390: protvirt: Do only reset registers that are accessible

Cornelia Huck
2020-02-25T13:06:52Z

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On Tue, 25 Feb 2020 13:51:12 +0100
Janosch Frank <frankja@linux.ibm.com> wrote:

> On 2/25/20 1:32 PM, Cornelia Huck wrote:
> > On Mon, 24 Feb 2020 06:40:55 -0500
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
> >
> >> From: Janosch Frank <frankja@linux.ibm.com>
> >>
> >> For protected VMs the hypervisor can not access guest breaking event
> >> address, program parameter, bpbc and todpr. Do not reset those fields
> >> as the control block does not provide access to these fields.
> >>
> >> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> >> Reviewed-by: David Hildenbrand <david@redhat.com>
> >> [borntraeger@de.ibm.com: patch merging, splitting, fixing]
> >> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> >> ---
> >> arch/s390/kvm/kvm-s390.c | 10 ++++++----
> >> 1 file changed, 6 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> >> index 6ab4c88f2e1d..c734e89235f9 100644
> >> --- a/arch/s390/kvm/kvm-s390.c
> >> +++ b/arch/s390/kvm/kvm-s390.c
> >> @@ -3499,14 +3499,16 @@ static void kvm_arch_vcpu_ioctl_initial_reset(struct kvm_vcpu *vcpu)
> >> kvm_s390_set_prefix(vcpu, 0);
> >> kvm_s390_set_cpu_timer(vcpu, 0);
> >> vcpu->arch.sie_block->ckc = 0;
> >> - vcpu->arch.sie_block->todpr = 0;
> >> memset(vcpu->arch.sie_block->gcr, 0, sizeof(vcpu->arch.sie_block->gcr));
> >> vcpu->arch.sie_block->gcr[0] = CR0_INITIAL_MASK;
> >> vcpu->arch.sie_block->gcr[14] = CR14_INITIAL_MASK;
> >> vcpu->run->s.regs.fpc = 0;
> >> - vcpu->arch.sie_block->gbea = 1;
> >> - vcpu->arch.sie_block->pp = 0;
> >> - vcpu->arch.sie_block->fpf &= ~FPF_BPBC;
> >> + if (!kvm_s390_pv_cpu_is_protected(vcpu)) {
> >> + vcpu->arch.sie_block->gbea = 1;
> >> + vcpu->arch.sie_block->pp = 0;
> >> + vcpu->arch.sie_block->fpf &= ~FPF_BPBC;
> >> + vcpu->arch.sie_block->todpr = 0;
> >
> > What happens if we do change those values? Is it just ignored or will
> > we get an exception on the next SIE entry?
>
> Well, changing gbea is a bad idea because of the sida overlay.
> I don't think that any other is checked, but I'd need to look up the
> todpr changes to be completely sure.

Maybe add a comment

/*
* Do not reset these registers in the protected case, as some of
* them are overlayed and they are not accessible in this case
* anyway.
*/

?

Just to avoid headscratching once this dropped out of our caches.

>
> >
> >> + }
> >> }
> >>
> >> static void kvm_arch_vcpu_ioctl_clear_reset(struct kvm_vcpu *vcpu)
> >
>
>

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