Messages in this thread | | | Date | Tue, 25 Jan 2022 13:00:27 +0100 | Subject | Re: [RFC PATCH v1 06/10] KVM: s390: Add vm IOCTL for key checked guest absolute memory access | From | Thomas Huth <> |
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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
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