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

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Date:   Thu, 19 Aug 2021 15:53:27 +0200
From:   Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        David Hildenbrand <david@...hat.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 8/18/21 10:06 AM, 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 :-)

fragment_len ? 
> 
>>
>>> +		rc = access_guest_frame(vcpu->kvm, mode, gpa, data, seg);
>>> +		len -= seg;
>>> +		gra += seg;
>>> +		data += seg;
>>>   	}
>>>   	return rc;
>>>   }
>>>
>>
>>
>