Re: [RFC 06/10] KVM: s390: Rework lowcore access functions

[Claudio Imbrenda <imbrenda@linux.ibm.com>]

On Mon, 16 Mar 2026 16:23:53 +0000
Janosch Frank <frankja@linux.ibm.com> wrote:

> These functions effectively always use offset constants and no
> addresses at all. Therefore make it clear that we're accessing offsets
> and sprinkle in compile and runtime warnings for more safety.
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
>  arch/s390/kvm/gaccess.h | 32 ++++++++++++++++++++++----------
>  1 file changed, 22 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/s390/kvm/gaccess.h b/arch/s390/kvm/gaccess.h
> index ee346b607a07..086da7b040b5 100644
> --- a/arch/s390/kvm/gaccess.h
> +++ b/arch/s390/kvm/gaccess.h
> @@ -89,6 +89,13 @@ static inline unsigned long kvm_s390_logical_to_effective(struct kvm_vcpu *vcpu,
>  	return _kvm_s390_logical_to_effective(&vcpu->arch.sie_block->gpsw, ga);
>  }
>  
> +static inline gpa_t lc_addr_from_offset(struct kvm_vcpu *vcpu, unsigned int off)

int is ok, but technically it could even be a short

> +{
> +	gpa_t addr = kvm_s390_get_prefix(vcpu);
> +
> +	return addr + off;
> +}
> +
>  /*
>   * put_guest_lc, read_guest_lc and write_guest_lc are guest access functions
>   * which shall only be used to access the lowcore of a vcpu.
> @@ -117,13 +124,14 @@ static inline unsigned long kvm_s390_logical_to_effective(struct kvm_vcpu *vcpu,
>   *	 would be to terminate the guest.
>   *	 It is wrong to inject a guest exception.
>   */
> -#define put_guest_lc(vcpu, x, gra)				\
> +#define put_guest_lc(vcpu, x, off)				\
>  ({								\
>  	struct kvm_vcpu *__vcpu = (vcpu);			\
> -	__typeof__(*(gra)) __x = (x);				\
> -	unsigned long __gpa;					\
> +	__typeof__(*(off)) __x = (x);				\
> +	gpa_t __gpa;						\
>  								\
> -	__gpa = (unsigned long)(gra);				\
> +	BUILD_BUG_ON(!__builtin_constant_p(off));		\
> +	__gpa = (unsigned long)(off);				\

why not use the new function lc_addr_from_offset() that you have just
introduced?

>  	__gpa += kvm_s390_get_prefix(__vcpu);			\
>  	kvm_write_guest(__vcpu->kvm, __gpa, &__x, sizeof(__x));	\
>  })
> @@ -131,7 +139,7 @@ static inline unsigned long kvm_s390_logical_to_effective(struct kvm_vcpu *vcpu,
>  /**
>   * write_guest_lc - copy data from kernel space to guest vcpu's lowcore
>   * @vcpu: virtual cpu
> - * @gra: vcpu's source guest real address
> + * @off: offset into the lowcore
>   * @data: source address in kernel space
>   * @len: number of bytes to copy
>   *
> @@ -146,18 +154,20 @@ static inline unsigned long kvm_s390_logical_to_effective(struct kvm_vcpu *vcpu,
>   *	 It is wrong to inject a guest exception.
>   */
>  static inline __must_check
> -int write_guest_lc(struct kvm_vcpu *vcpu, unsigned long gra, void *data,
> +int write_guest_lc(struct kvm_vcpu *vcpu, unsigned int off, void *data,
>  		   unsigned long len)
>  {
> -	unsigned long gpa = gra + kvm_s390_get_prefix(vcpu);
> +	gpa_t gpa = lc_addr_from_offset(vcpu, off);
>  
> +	BUILD_BUG_ON(!__builtin_constant_p(off) || !__builtin_constant_p(len));
> +	BUILD_BUG_ON(off + len >= 2 * PAGE_SIZE);
>  	return kvm_write_guest(vcpu->kvm, gpa, data, len);
>  }
>  
>  /**
>   * read_guest_lc - copy data from guest vcpu's lowcore to kernel space
>   * @vcpu: virtual cpu
> - * @gra: vcpu's source guest real address
> + * @off: offset into the lowcore
>   * @data: destination address in kernel space
>   * @len: number of bytes to copy
>   *
> @@ -172,11 +182,13 @@ int write_guest_lc(struct kvm_vcpu *vcpu, unsigned long gra, void *data,
>   *	 It is wrong to inject a guest exception.
>   */
>  static inline __must_check
> -int read_guest_lc(struct kvm_vcpu *vcpu, unsigned long gra, void *data,
> +int read_guest_lc(struct kvm_vcpu *vcpu, unsigned int off, void *data,
>  		  unsigned long len)
>  {
> -	unsigned long gpa = gra + kvm_s390_get_prefix(vcpu);
> +	gpa_t gpa = lc_addr_from_offset(vcpu, off);
>  
> +	BUILD_BUG_ON(!__builtin_constant_p(off) || !__builtin_constant_p(len));
> +	BUILD_BUG_ON(off + len >= 2 * PAGE_SIZE);
>  	return kvm_read_guest(vcpu->kvm, gpa, data, len);
>  }
>