Re: [PATCH v1 1/2] s390x: KVM: accept STSI for CPU topology information

[David Hildenbrand <david@redhat.com>]

On 14.07.21 17:25, Pierre Morel wrote:
> STSI(15.1.x) gives information on the CPU configuration topology.
> Let's accept the interception of STSI with the function code 15 and
> let the userland part of the hypervisor handle it.
> 
> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> ---
>   arch/s390/kvm/priv.c | 11 ++++++++++-
>   1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
> index 9928f785c677..4ab5f8b7780e 100644
> --- a/arch/s390/kvm/priv.c
> +++ b/arch/s390/kvm/priv.c
> @@ -856,7 +856,7 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
>   	if (vcpu->arch.sie_block->gpsw.mask & PSW_MASK_PSTATE)
>   		return kvm_s390_inject_program_int(vcpu, PGM_PRIVILEGED_OP);
>   
> -	if (fc > 3) {
> +	if (fc > 3 && fc != 15) {
>   		kvm_s390_set_psw_cc(vcpu, 3);
>   		return 0;
>   	}
> @@ -893,6 +893,15 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
>   			goto out_no_data;
>   		handle_stsi_3_2_2(vcpu, (void *) mem);
>   		break;
> +	case 15:
> +		if (sel1 != 1 || sel2 < 2 || sel2 > 6)
> +			goto out_no_data;
> +		if (vcpu->kvm->arch.user_stsi) {
> +			insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);
> +			return -EREMOTE;
> +		}
> +		kvm_s390_set_psw_cc(vcpu, 3);
> +		return 0;
>   	}
>   	if (kvm_s390_pv_cpu_is_protected(vcpu)) {
>   		memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
> 

1. Setting GPRS to 0

I was wondering why we have the "vcpu->run->s.regs.gprs[0] = 0;"
for existing fc 1,2,3 in case we set cc=0.

Looking at the doc, all I find is:

"CC 0: Requested configuration-level number placed in
general register 0 or requested SYSIB informa-
tion stored"

But I don't find where it states that we are supposed to set
general register 0 to 0. Wouldn't we also have to do it for
fc=15 or for none?

If fc 1,2,3 and 15 are to be handled equally, I suggest the following:

diff --git a/arch/s390/kvm/priv.c b/arch/s390/kvm/priv.c
index 9928f785c677..6eb86fa58b0b 100644
--- a/arch/s390/kvm/priv.c
+++ b/arch/s390/kvm/priv.c
@@ -893,17 +893,23 @@ static int handle_stsi(struct kvm_vcpu *vcpu)
                         goto out_no_data;
                 handle_stsi_3_2_2(vcpu, (void *) mem);
                 break;
+       case 15:
+               if (sel1 != 1 || sel2 < 2 || sel2 > 6)
+                       goto out_no_data;
+               break;
         }
-       if (kvm_s390_pv_cpu_is_protected(vcpu)) {
-               memcpy((void *)sida_origin(vcpu->arch.sie_block), (void *)mem,
-                      PAGE_SIZE);
-               rc = 0;
-       } else {
-               rc = write_guest(vcpu, operand2, ar, (void *)mem, PAGE_SIZE);
-       }
-       if (rc) {
-               rc = kvm_s390_inject_prog_cond(vcpu, rc);
-               goto out;
+       if (mem) {
+               if (kvm_s390_pv_cpu_is_protected(vcpu)) {
+                       memcpy((void *)sida_origin(vcpu->arch.sie_block),
+                              (void *)mem, PAGE_SIZE);
+               } else {
+                       rc = write_guest(vcpu, operand2, ar, (void *)mem,
+                                        PAGE_SIZE);
+                       if (rc) {
+                               rc = kvm_s390_inject_prog_cond(vcpu, rc);
+                               goto out;
+                       }
+               }
         }
         if (vcpu->kvm->arch.user_stsi) {
                 insert_stsi_usr_data(vcpu, operand2, ar, fc, sel1, sel2);


2. maximum-MNest facility

"
1. If the maximum-MNest facility is installed and
selector 2 exceeds the nonzero model-depen-
dent maximum-selector-2 value."

2. If the maximum-MNest facility is not installed and
selector 2 is not specified as two.
"

We will we be handling the presence/absence of the maximum-MNest facility
(for our guest?) in QEMU, corect?

I do wonder if we should just let any fc=15 go to user space let the whole
sel1 / sel2 checking be handled there. I don't think it's a fast path after all.
But no strong opinion.

How do we identify availability of maximum-MNest facility?


3. User space awareness

How can user space identify that we actually forward these intercepts?
How can it enable them? The old KVM_CAP_S390_USER_STSI capability
is not sufficient.

I do wonder if we want KVM_CAP_S390_USER_STSI_15 or sth like that to change
the behavior once enabled by user space.


4. Without vcpu->kvm->arch.user_stsi, we indicate cc=0 to our guest,
also for fc 1,2,3. Is that actually what we want? (or do we simply not care
because the guest is not supposed to use stsi?)

-- 
Thanks,

David / dhildenb