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

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Date:   Wed, 18 Aug 2021 10:06:31 +0200
From:   Janosch Frank <frankja@...ux.ibm.com>
To:     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 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 :-)

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