Re: [PATCH v7 12/21] KVM: s390: vsie: Add helper unshadow_intercept()

[Janosch Frank <frankja@linux.ibm.com>]

On 9/18/26 5:09 PM, Christoph Schlameuss wrote:
> Extract helper method from existing code to reuse in new code.
> 
> unshadow_intercept() acts like unshadow_scb() but only copies intercept
> information. Used where the rest of the scb is not yet shadowed.
> 
> Signed-off-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
> ---
>   arch/s390/kvm/s390/vsie.c | 28 ++++++++++++++++++++--------
>   1 file changed, 20 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/s390/kvm/s390/vsie.c b/arch/s390/kvm/s390/vsie.c
> index 95baac9a1ccb..c42e2df4c0ab 100644
> --- a/arch/s390/kvm/s390/vsie.c
> +++ b/arch/s390/kvm/s390/vsie.c
> @@ -565,18 +565,27 @@ static void prepare_ibc(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
>   	}
>   }
>   
> -/* unshadow the scb, copying parameters back to the real scb */
> -static void unshadow_scb(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page)
> +/*
> + * Only write the interception fields back; avoid clobbering the timer and
> + * guest-state fields in scb_o.
> + */
> +static inline void unshadow_intercept(struct kvm_s390_sie_block *scb_o,
> +				      struct kvm_s390_sie_block *scb_s)
>   {
> -	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
> -	struct kvm_s390_sie_block *scb_o = vsie_page->scb_o;
> -
> -	/* interception */
>   	scb_o->icptcode = scb_s->icptcode;
>   	scb_o->icptstatus = scb_s->icptstatus;
>   	scb_o->ipa = scb_s->ipa;
>   	scb_o->ipb = scb_s->ipb;
>   	scb_o->gbea = scb_s->gbea;
I have the feeling that we make things difficult for ourselves by not 
writing SIE entry validities into the scb_o directly. And I think you 
somewhat recognized that problem since you chose to write the validity 
to scb_o and not scb_s in _shadow_sca_cpu() but I have yet to fully read 
that patch.

For 99% of the validities we've never even entered SIE so we copy data 
twice for absolutely no reason other than symmetry. out_unshadow in 
kvm_s390_handle_vsie() isn't really necessary, if pin_blocks() fails we 
can jump to unpin_scb() if we write the validity to scb_o.

Maybe we could move (un)shadow_scb() into vsie_run() and be done with it.

I have some more reading to do.