linux-kernel - Re: [RFC PATCH v1 06/10] KVM: s390: Add vm IOCTL for key checked guest absolute memory access

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Date:   Thu, 20 Jan 2022 11:38:29 +0100
From:   Thomas Huth <thuth@...hat.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 06/10] KVM: s390: Add vm IOCTL for key checked
 guest absolute memory access

On 18/01/2022 10.52, Janis Schoetterl-Glausch wrote:
> Channel I/O honors storage keys and is performed on absolute memory.
> For I/O emulation user space therefore needs to be able to do key
> checked accesses.
> The vm IOCTL supports read/write accesses, as well as checking
> if an access would succeed.
...
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index e3f450b2f346..dd04170287fd 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -572,6 +572,8 @@ struct kvm_s390_mem_op {
>   #define KVM_S390_MEMOP_LOGICAL_WRITE	1
>   #define KVM_S390_MEMOP_SIDA_READ	2
>   #define KVM_S390_MEMOP_SIDA_WRITE	3
> +#define KVM_S390_MEMOP_ABSOLUTE_READ	4
> +#define KVM_S390_MEMOP_ABSOLUTE_WRITE	5

Not quite sure about this - maybe it is, but at least I'd like to see this 
discussed: Do we really want to re-use the same ioctl layout for both, the 
VM and the VCPU file handles? Where the userspace developer has to know that 
the *_ABSOLUTE_* ops only work with VM handles, and the others only work 
with the VCPU handles? A CPU can also address absolute memory, so why not 
adding the *_ABSOLUTE_* ops there, too? And if we'd do that, wouldn't it be 
sufficient to have the VCPU ioctls only - or do you want to call these 
ioctls from spots in QEMU where you do not have a VCPU handle available? 
(I/O instructions are triggered from a CPU, so I'd assume that you should 
have a VCPU handle around?)

  Thomas