Messages in this thread | | | Date | Tue, 22 Nov 2022 08:47:17 +0100 | From | Thomas Huth <> | Subject | Re: [PATCH v3 2/9] Documentation: KVM: s390: Describe KVM_S390_MEMOP_F_CMPXCHG |
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On 17/11/2022 23.17, Janis Schoetterl-Glausch wrote: > Describe the semantics of the new KVM_S390_MEMOP_F_CMPXCHG flag for > absolute vm write memops which allows user space to perform (storage key > checked) cmpxchg operations on guest memory. > > Signed-off-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com> > --- ... > Supported flags: > * ``KVM_S390_MEMOP_F_CHECK_ONLY`` > * ``KVM_S390_MEMOP_F_SKEY_PROTECTION`` > + * ``KVM_S390_MEMOP_F_CMPXCHG`` > + > +The semantics of the flags common with logical acesses are as for logical > +accesses. > + > +For write accesses, the KVM_S390_MEMOP_F_CMPXCHG might be supported.
I'd maybe merge this with the last sentence:
For write accesses, the KVM_S390_MEMOP_F_CMPXCHG flag is supported if KVM_CAP_S390_MEM_OP_EXTENSION has bit 1 (i.e. bit with value 2) set.
... and speaking of that, I wonder whether it's maybe a good idea to introduce some #defines for bit 1 / value 2, to avoid the confusion ?
> +In this case, instead of doing an unconditional write, the access occurs only > +if the target location contains the "size" byte long value pointed to by > +"old_p". This is performed as an atomic cmpxchg.
I had to read the first sentence twice to understand it ... maybe it's easier to understand if you move the "size" part to the second sentence:
In this case, instead of doing an unconditional write, the access occurs only if the target location contains value pointed to by "old_p". This is performed as an atomic cmpxchg with the length specified by the "size" parameter.
?
> "size" must be a power of two > +up to and including 16. > +The value at the target location is written to the location "old_p" points to.
IMHO something like this would be better:
The value at the target location is replaced with the value from the location that "old_p" points to.
> +If the exchange did not take place because the target value doesn't match the > +old value KVM_S390_MEMOP_R_NO_XCHG is returned. > +The KVM_S390_MEMOP_F_CMPXCHG flag is supported if KVM_CAP_S390_MEM_OP_EXTENSION > +has bit 1 (i.e. bit with value 2) set.
Thomas
PS: Please take my suggestions with a grain of salt ... I'm not a native speaker either.
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