Re: [PATCH v4 18/20] KVM: s390: vsie: Add VSIE shadow stat counters

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] The `vsie_shadow_scb` statistic undercounts allocated shadow SCB pages if their initialization fails, and incorrectly categorizes subsequent uses as reuses.
--

--- Patch [18]: [PATCH v4 18/20] KVM: s390: vsie: Add VSIE shadow stat counters ---
commit 30e90ae4cab73ef72cfd191918cda52ff46f4c0c
Author: Christoph Schlameuss <[email protected]>

KVM: s390: vsie: Add VSIE shadow stat counters

This commit adds new statistics counters to track the allocation and reuse of
VSIE shadow SCBs and SCAs.

> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
> index 5f6dd4b9e9fd7..a1b679b4aa361 100644
> --- a/arch/s390/kvm/vsie.c
> +++ b/arch/s390/kvm/vsie.c

[ ... ]

> @@ -2042,6 +2047,7 @@ static struct vsie_page *get_vsie_page_cpu_nr(struct kvm_vcpu *vcpu, struct vsie
>  		}
>  	}
>  	if (vsie_page != vsie_page_new) {
> +		vsie_page_is_new = false;
>  		if (vsie_page_new)
>  			free_vsie_page(vsie_page_new);
>  
> @@ -2059,6 +2065,11 @@ static struct vsie_page *get_vsie_page_cpu_nr(struct kvm_vcpu *vcpu, struct vsie
>  			put_vsie_page(vsie_page);
>  			return ERR_PTR(rc);
>  		}
> +
> +		if (vsie_page_is_new)
> +			vcpu->kvm->stat.vsie_shadow_scb++;
> +		else
> +			vcpu->kvm->stat.vsie_shadow_scb_reuse++;
>  	}
>  
>  	return vsie_page;

[Severity: Medium]
Will this lead to an undercount of vsie_shadow_scb allocations if
vsie_page_init() fails?

If vsie_page_init() fails in get_vsie_page_cpu_nr(), it returns ERR_PTR(rc)
and skips the stat increment here.

However, the newly allocated page is already saved in vsie_sca->pages[cpu_nr]
earlier in the function. The next time this page is fetched from the array,
vsie_page_is_new will evaluate to false, causing vsie_shadow_scb_reuse to be
incremented instead of counting the initial allocation.

Should the vsie_shadow_scb counter be incremented earlier upon successful
allocation, or should the page be removed from vsie_sca->pages[cpu_nr] on
error?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/[email protected]?part=18