linux-kernel - Re: [PATCH 1/2] KVM: s390: gaccess: Cleanup access to guest frames

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Message-ID: <0c800ed3-cc60-9c13-c0a7-8ba302365ccb@redhat.com>
Date:   Wed, 18 Aug 2021 10:44:34 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        kvm@...r.kernel.org, borntraeger@...ibm.com,
        imbrenda@...ux.ibm.com, Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     cohuck@...hat.com, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] KVM: s390: gaccess: Cleanup access to guest frames

On 18.08.21 10:06, Janosch Frank wrote:
> On 8/18/21 9:54 AM, David Hildenbrand wrote:
>> On 16.08.21 17:07, Janis Schoetterl-Glausch wrote:
>>> Introduce a helper function for guest frame access.
>>> Rewrite the calculation of the gpa and the length of the segment
>>> to be more readable.
>>>
>>> Signed-off-by: Janis Schoetterl-Glausch <scgl@...ux.ibm.com>
> [...]
>>> -	unsigned long _len, gpa;
>>> +	unsigned long gpa;
>>> +	unsigned int seg;
>>>    	int rc = 0;
>>>    
>>>    	while (len && !rc) {
>>>    		gpa = kvm_s390_real_to_abs(vcpu, gra);
>>> -		_len = min(PAGE_SIZE - (gpa & ~PAGE_MASK), len);
>>> -		if (mode)
>>> -			rc = write_guest_abs(vcpu, gpa, data, _len);
>>> -		else
>>> -			rc = read_guest_abs(vcpu, gpa, data, _len);
>>> -		len -= _len;
>>> -		gra += _len;
>>> -		data += _len;
>>> +		seg = min(PAGE_SIZE - offset_in_page(gpa), len);
>>
>> What does "seg" mean? I certainly know when "len" means -- which is also
>> what the function eats.
> 
> What does "_len" mean especially in contrast to "len"?
> 
> "seg" is used in the common kvm guest access functions so it's at least
> consistent although I share the sentiment that it's not a great name for
> the length we access inside the page.
> 
> Originally I suggested "len_in_page" if you have a better name I'd
> expect we'll both be happy to discuss it :-)

Similar code I encountered in other places uses "len" vs "cur_len" or 
"total_len" vs. "cur_len". I agree that everything is better than "len" 
vs. "_len".

I just cannot come up with a proper word for "seg" that would make 
sense. "Segment" ? Maybe my uneducated mind is missing some important 
English words that just fit perfectly here.

Anyhow, just my 2 cents, you maintain this code :)

-- 
Thanks,

David / dhildenb