linux-kernel - Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key checked cmpxchg

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Date:   Fri, 2 Dec 2022 10:00:28 +0100
From:   Claudio Imbrenda <imbrenda@...ux.ibm.com>
To:     Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
Cc:     Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        David Hildenbrand <david@...hat.com>,
        Jonathan Corbet <corbet@....net>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org, linux-s390@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Sven Schnelle <svens@...ux.ibm.com>
Subject: Re: [PATCH v3 1/9] KVM: s390: Extend MEM_OP ioctl by storage key
 checked cmpxchg

On Thu, 01 Dec 2022 18:44:56 +0100
Janis Schoetterl-Glausch <scgl@...ux.ibm.com> wrote:

> > 
> > please also document -EOPNOTSUPP  
> 
> I'd add "* -EOPNOTSUPP: should never occur", then, that ok with you?

no, also explain in which conditions it is returned

something like: 
 * -EOPNOTSUPP: if the memslot is not writable (should never occour)

> >   
> > > + */
> > > +int cmpxchg_guest_abs_with_key(struct kvm *kvm, gpa_t gpa, int len,
> > > +			       __uint128_t *old_p, __uint128_t new,
> > > +			       u8 access_key)
> > > +{
> > > +	gfn_t gfn = gpa >> PAGE_SHIFT;
> > > +	struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);  
> > 
> > exchange the above two lines (reverse christmas tree)  
> 
> Is this a hard requirement? Since there is a dependency.
> If I do the initialization further down, the order wouldn't actually change.

ahhhhh right, I had missed that

keep it as it is, of course

[...]

> > I really dislike repeating the same code 5 times, but I guess there was
> > no other way?  
> 
> I could use the function called by cmpxchg_user_key directly, but Heiko won't agree to that.
> A macro would work too, of course, not sure if I prefer that tho.

ok so there is no other way, let's keep it as it is

[...]

> To me it feels like KVM_S390_MEMOP_R_NO_XCHG is api surface and should be referenced here.
> cmpxchg_guest_abs_with_key isn't mem op specific
> (of course that's the only thing it is currently used for).

fair enough

> >   
> > > +				if (copy_to_user(old_p, &old.raw[off_in_quad], mop->size))
> > > +					r = -EFAULT;
> > > +			}
> > >  		} else {
> > >  			if (copy_from_user(tmpbuf, uaddr, mop->size)) {
> > >  				r = -EFAULT;  
> >   
>