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:   Tue, 25 Jan 2022 13:00:27 +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 20/01/2022 13.23, Janis Schoetterl-Glausch wrote:
> On 1/20/22 11:38, Thomas Huth wrote:
>> 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?)
> 
> There are some differences between the vm and the vcpu memops.
> No storage or fetch protection overrides apply to IO/vm memops, after all there is no control register to enable them.
> Additionally, quiescing is not required for IO, tho in practice we use the same code path for the vcpu and the vm here.
> Allowing absolute accesses with a vcpu is doable, but I'm not sure what the use case for it would be, I'm not aware of
> a precedence in the architecture. Of course the vcpu memop already supports logical=real accesses.

Ok. Maybe it then would be better to call new ioctl and the new op defines 
differently, to avoid confusion? E.g. call it "vmmemop" and use:

#define KVM_S390_VMMEMOP_ABSOLUTE_READ    1
#define KVM_S390_VMMEMOP_ABSOLUTE_WRITE   2

?

  Thomas