Re: [RFCv2 21/37] KVM: S390: protvirt: Introduce instruction data area bounce buffer

[Thomas Huth <thuth@redhat.com>]

On 03/02/2020 14.19, Christian Borntraeger wrote:
> From: Janosch Frank <frankja@linux.ibm.com>
> 
> Now that we can't access guest memory anymore, we have a dedicated
> sattelite block that's a bounce buffer for instruction data.
> 
> We re-use the memop interface to copy the instruction data to / from
> userspace. This lets us re-use a lot of QEMU code which used that
> interface to make logical guest memory accesses which are not possible
> anymore in protected mode anyway.
> 
> Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
> ---
[...]
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index eab741bc12c3..20969ce12096 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -466,7 +466,7 @@ struct kvm_translation {
>  	__u8  pad[5];
>  };
>  
> -/* for KVM_S390_MEM_OP */
> +/* for KVM_S390_MEM_OP and KVM_S390_SIDA_OP */
>  struct kvm_s390_mem_op {
>  	/* in */
>  	__u64 gaddr;		/* the guest address */
> @@ -475,11 +475,17 @@ struct kvm_s390_mem_op {
>  	__u32 op;		/* type of operation */
>  	__u64 buf;		/* buffer in userspace */
>  	__u8 ar;		/* the access register number */
> -	__u8 reserved[31];	/* should be set to 0 */
> +	__u8 reserved21[3];	/* should be set to 0 */
> +	__u32 offset;		/* offset into the sida */
> +	__u8 reserved28[24];	/* should be set to 0 */
>  };
> +
> +
>  /* types for kvm_s390_mem_op->op */
>  #define KVM_S390_MEMOP_LOGICAL_READ	0
>  #define KVM_S390_MEMOP_LOGICAL_WRITE	1
> +#define KVM_S390_MEMOP_SIDA_READ	2
> +#define KVM_S390_MEMOP_SIDA_WRITE	3
>  /* flags for kvm_s390_mem_op->flags */
>  #define KVM_S390_MEMOP_F_CHECK_ONLY		(1ULL << 0)
>  #define KVM_S390_MEMOP_F_INJECT_EXCEPTION	(1ULL << 1)
> @@ -1510,6 +1516,7 @@ struct kvm_pv_cmd {
>  /* Available with KVM_CAP_S390_PROTECTED */
>  #define KVM_S390_PV_COMMAND		_IOW(KVMIO, 0xc5, struct kvm_pv_cmd)
>  #define KVM_S390_PV_COMMAND_VCPU	_IOW(KVMIO, 0xc6, struct kvm_pv_cmd)
> +#define KVM_S390_SIDA_OP		_IOW(KVMIO, 0xc7, struct kvm_s390_mem_op)
>  
>  /* Secure Encrypted Virtualization command */
>  enum sev_cmd_id {
> 

Uh, why the mix of a new ioctl with the existing mem_op stuff? Could you
please either properly integrate this into the MEM_OP ioctl (and e.g.
use gaddr as offset for the new SIDA_READ and SIDA_WRITE subcodes), or
completely separate it for a new ioctl, i.e. introduce a new struct for
the new ioctl instead of recycling the struct kvm_s390_mem_op here?
(and in case you ask me, I'd slightly prefer to integrate everything
into MEM_OP instead of introducing a new ioctl here).

In any case, please also update Documentation/virt/kvm/api.txt with the
new SIDA functions.

 Thanks,
  Thomas