Re: [PATCH] KVM: s390: fix validity interception issue when gisa is switched off

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

On 7/31/24 1:31 PM, Michael Mueller wrote:
> The following validity interception occures when the gisa usage has been
> switched off either by using kernel parameter "kvm.use_gisa=0" or by
> setting the related sysfs attribute to N (echo N >/sys/module/kvm/
> parameters/use_gisa).
> 
> The issue surfaces in the host kernel with the following kernel message as
> soon a new kvm guest start has been attemted.

How about:

We might run into a SIE validity if gisa has been disabled either via 
using kernel parameter "kvm.use_gisa=0" or by setting the related sysfs 
attribute to N (echo N >/sys/module/kvm/parameters/use_gisa).

The validity is caused by an invalid value in the SIE control block's 
gisa designation. That happens because we pass the uninitialized gisa 
origin to virt_to_phys() before writing it to the gisa designation.

To fix this we return 0 in kvm_s390_get_gisa_desc() if the origin is 0. 
kvm_s390_get_gisa_desc() is used to determine which gisa designation to 
set in the SIE control block. A value of 0 in the gisa designation 
disables gisa usage.


> Cc: stable@vger.kernel.org
> Reported-by: Christian Borntraeger <borntraeger@linux.ibm.com>

> Closes: https://ibm-systems-z.slack.com/archives/C04BWBXSKEY/p1722280755665409

I'd just remove that since it's not accessible for people outside of 
IBM. Yes, checkpatch will complain but in this case it's ok to ignore that.

> Fixes: fe0ef0030463 ("KVM: s390: sort out physical vs virtual pointers usage")
> Signed-off-by: Michael Mueller <mimu@linux.ibm.com>
> ---
>   arch/s390/kvm/kvm-s390.h | 7 ++++++-
>   1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/kvm/kvm-s390.h b/arch/s390/kvm/kvm-s390.h
> index bf8534218af3..e680c6bf0c9d 100644
> --- a/arch/s390/kvm/kvm-s390.h
> +++ b/arch/s390/kvm/kvm-s390.h
> @@ -267,7 +267,12 @@ static inline unsigned long kvm_s390_get_gfn_end(struct kvm_memslots *slots)
>   
>   static inline u32 kvm_s390_get_gisa_desc(struct kvm *kvm)
>   {
> -	u32 gd = virt_to_phys(kvm->arch.gisa_int.origin);
> +	u32 gd;
> +
> +	if (!kvm->arch.gisa_int.origin)
> +		return 0;
> +
> +	gd = virt_to_phys(kvm->arch.gisa_int.origin);
>   
>   	if (gd && sclp.has_gisaf)
>   		gd |= GISA_FORMAT1;