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 12:20:02 +0100
From:   Christian Borntraeger <borntraeger@...ux.ibm.com>
To:     Thomas Huth <thuth@...hat.com>,
        Janis Schoetterl-Glausch <scgl@...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



Am 20.01.22 um 11:38 schrieb Thomas Huth:
> 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?)

I paritally agree. the ABSOLUTE calls should also work from a VCPU handle. But you also want to be able to call this from a different thread than the vpcu threads as you do not want to kick a CPU out of the kernel just for that. So it makes sense to have this ioctl also for the VM fd.
> 
>   Thomas
> 
>