Re: [PATCH RFC v2 08/11] KVM: s390: Allow guest-3 cpu add and remove with vsie sigpif

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

On 11/10/25 18:16, Christoph Schlameuss wrote:
> As we are shadowing the SCA we need to add and remove the pointers to
> the shadowed control blocks and sca entries whenever the mcn changes.
> 
> It is not expected that the mcn changes frequently for a already running
> guest-3 configuration. So we can simply re-init the ssca whenever the
> mcn changes.
> To detect the mcn change we store the expected mcn in the struct
> vsie_sca when running the ssca init.
> 
> Signed-off-by: Christoph Schlameuss <schlameuss@linux.ibm.com>
> ---
>   arch/s390/kvm/vsie.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 72c794945be916cc107aba74e1609d3b4780d4b9..1e15220e1f1ecfd83b10aa0620ca84ff0ff3c1ac 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c
> @@ -1926,12 +1926,27 @@ static int init_ssca(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page, struct
>   	return PTR_ERR(vsie_page_n);
>   }
>   
> +static bool sca_mcn_equals(struct vsie_sca *sca, u64 *mcn)
> +{
> +	bool is_esca = test_bit(VSIE_SCA_ESCA, &sca->flags);
> +	int i;
> +
> +	if (!is_esca)
> +		return ((struct bsca_block *)phys_to_virt(sca->ssca->osca))->mcn == *mcn;
> +
> +	for (i = 0; i < 4; i++)
> +		if (((struct esca_block *)phys_to_virt(sca->ssca->osca))->mcn[i] != sca->mcn[i])
> +			return false;

You're reimplementing memcmp(), no?
Instead of casting which makes the comparison really messy you could use 
offsetof.

Something like this (+- errors):

void *osca = phys_to_virt(sca->ssca->osca);
int offset = offsetof(struct bsca_block, mcn);
int size = 8;

if (test_bit(VSIE_SCA_ESCA, &sca->flags)) {
	size = 8 * 4;
	offset = offsetof(struct esca_block, mcn);
}

return !memcmp(osca + offset, mcn, size);


> +	return true;
> +}
> +
>   /*
>    * Shadow the sca on vsie enter.
>    */
>   static int shadow_sca(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page, struct vsie_sca *sca)
>   {
>   	struct kvm_s390_sie_block *scb_s = &vsie_page->scb_s;
> +	bool do_init_ssca;
>   	int rc;
>   
>   	vsie_page->sca = sca;
> @@ -1947,8 +1962,9 @@ static int shadow_sca(struct kvm_vcpu *vcpu, struct vsie_page *vsie_page, struct
>   	if (!use_vsie_sigpif_for(vcpu->kvm, vsie_page))
>   		return false;
>   
> -	/* skip if the guest does not have an usable sca */
> -	if (!sca->ssca->osca) {
> +	do_init_ssca = !sca->ssca->osca;
> +	do_init_ssca = do_init_ssca || !sca_mcn_equals(sca, sca->mcn);
> +	if (do_init_ssca) {
>   		rc = init_ssca(vcpu, vsie_page, sca);
>   		if (rc)
>   			return rc;
>