linux-kernel - Re: [PATCH v6 08/14] KVM: s390: Move common code of mem

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Date:   Thu, 26 Jan 2023 17:47:43 +0100
From:   Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Thomas Huth <thuth@...hat.com>,
        Christian Borntraeger <borntraeger@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>
Cc:     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 v6 08/14] KVM: s390: Move common code of mem_op
 functions into functions

On Thu, 2023-01-26 at 14:02 +0100, Janosch Frank wrote:
> On 1/26/23 07:48, Thomas Huth wrote:
> > On 25/01/2023 22.26, Janis Schoetterl-Glausch wrote:
> > > The vcpu and vm mem_op ioctl implementations share some functionality.
> > > Move argument checking and buffer allocation into functions and call
> > > them from both implementations.
> > > This allows code reuse in case of additional future mem_op operations.
> > > 
> > > Suggested-by: Janosch Frank <frankja@...ux.ibm.com>
> > > Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> > > ---
> > >    arch/s390/kvm/kvm-s390.c | 80 +++++++++++++++++++++-------------------
> > >    1 file changed, 42 insertions(+), 38 deletions(-)
> > > 
> > > diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
> > > index e4890e04b210..e0dfaa195949 100644
> > > --- a/arch/s390/kvm/kvm-s390.c
> > > +++ b/arch/s390/kvm/kvm-s390.c
> > > @@ -2764,24 +2764,44 @@ static int kvm_s390_handle_pv(struct kvm *kvm, struct kvm_pv_cmd *cmd)
> > >    	return r;
> > >    }
> > >    
> > > -static bool access_key_invalid(u8 access_key)
> > > +static int mem_op_validate_common(struct kvm_s390_mem_op *mop, u64 supported_flags)
> > >    {
> > > -	return access_key > 0xf;
> > > +	if (mop->flags & ~supported_flags || !mop->size)
> > > +		return -EINVAL;
> > > +	if (mop->size > MEM_OP_MAX_SIZE)
> > > +		return -E2BIG;
> > > +	if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
> > > +		if (mop->key > 0xf)
> > > +			return -EINVAL;
> > > +	} else {
> > > +		mop->key = 0;
> > > +	}
> > > +	return 0;
> > > +}
> > > +
> > > +static void *mem_op_alloc_buf(struct kvm_s390_mem_op *mop)
> > > +{
> > > +	void *buf;
> > > +
> > > +	if (mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)
> > > +		return NULL;
> > > +	buf = vmalloc(mop->size);
> > > +	if (!buf)
> > > +		return ERR_PTR(-ENOMEM);
> > > +	return buf;
> > >    }
> > >    
> > >    static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> > >    {
> > >    	void __user *uaddr = (void __user *)mop->buf;
> > > -	u64 supported_flags;
> > >    	void *tmpbuf = NULL;
> > 
> > You likely can now remove the "= NULL" here, I guess?
> > 
> > >    	int r, srcu_idx;
> > >    
> > > -	supported_flags = KVM_S390_MEMOP_F_SKEY_PROTECTION
> > > -			  | KVM_S390_MEMOP_F_CHECK_ONLY;
> > > -	if (mop->flags & ~supported_flags || !mop->size)
> > > -		return -EINVAL;
> > > -	if (mop->size > MEM_OP_MAX_SIZE)
> > > -		return -E2BIG;
> > > +	r = mem_op_validate_common(mop, KVM_S390_MEMOP_F_SKEY_PROTECTION |
> > > +					KVM_S390_MEMOP_F_CHECK_ONLY);
> > > +	if (r)
> > > +		return r;
> > > +
> > >    	/*
> > >    	 * This is technically a heuristic only, if the kvm->lock is not
> > >    	 * taken, it is not guaranteed that the vm is/remains non-protected.
> > > @@ -2793,17 +2813,9 @@ static int kvm_s390_vm_mem_op(struct kvm *kvm, struct kvm_s390_mem_op *mop)
> > >    	 */
> > >    	if (kvm_s390_pv_get_handle(kvm))
> > >    		return -EINVAL;
> > > -	if (mop->flags & KVM_S390_MEMOP_F_SKEY_PROTECTION) {
> > > -		if (access_key_invalid(mop->key))
> > > -			return -EINVAL;
> > > -	} else {
> > > -		mop->key = 0;
> > > -	}
> > > -	if (!(mop->flags & KVM_S390_MEMOP_F_CHECK_ONLY)) {
> > > -		tmpbuf = vmalloc(mop->size);
> > > -		if (!tmpbuf)
> > > -			return -ENOMEM;
> > > -	}
> > > +	tmpbuf = mem_op_alloc_buf(mop);
> > > +	if (IS_ERR(tmpbuf))
> > > +		return PTR_ERR(tmpbuf);
> > >    
> > >    	srcu_idx = srcu_read_lock(&kvm->srcu);
> > >    
> > > @@ -5250,28 +5262,20 @@ static long kvm_s390_vcpu_mem_op(struct kvm_vcpu *vcpu,
> > >    {
> > >    	void __user *uaddr = (void __user *)mop->buf;
> > >    	void *tmpbuf = NULL;
> > 
> > ... and here, too.
> > 
> > But I have to admit that I'm also not sure whether I like the
> > mem_op_alloc_buf() part or not (the mem_op_validate_common() part looks fine
> > to me) : mem_op_alloc_buf() is a new function with 11 lines of code, and the
> > old spots that allocate memory were only 5 lines of code each, so you now
> > increased the LoC count and additionally have to fiddly with IS_ERR and
> > PTR_ERR which is always a little bit ugly in my eyes ... IMHO I'd rather
> > keep the old code here. But that's just my 0.02 €, if you think it's nicer
> > with mem_op_alloc_buf(), I won't insist on keeping the old code.
> > 
> >    Thomas
> > 
> 
> I've done a PoC that has a **buff argument and combines the check with 
> the alloc.

I just didn't like that much because it felt like an unspecific memop_do_things function.