linux-kernel - Re: [PATCH v2 3/3] KVM: s390: gaccess: Cleanup access to guest frames

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Date:   Mon, 22 Nov 2021 12:13:24 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Janosch Frank <frankja@...ux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.vnet.ibm.com>,
        Janis Schoetterl-Glausch <scgl@...ux.ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] KVM: s390: gaccess: Cleanup access to guest frames

On 19.11.21 10:00, Janosch Frank wrote:
> On 10/28/21 16:48, Janis Schoetterl-Glausch wrote:
>> On 10/28/21 16:25, David Hildenbrand wrote:
>>> On 28.10.21 15:55, Janis Schoetterl-Glausch wrote:
>>>> Introduce a helper function for guest frame access.
>>>
>>> "guest page access"
>>
>> Ok.
>>>
>>> But I do wonder if you actually want to call it
>>>
>>> "access_guest_abs"
>>>
>>> and say "guest absolute access" instead here.
>>>
>>> Because we're dealing with absolute addresses and the fact that we are
>>> accessing it page-wise is just because we have to perform a page-wise
>>> translation in the callers (either virtual->absolute or real->absolute).
>>>
>>> Theoretically, if you know you're across X pages but they are contiguous
>>> in absolute address space, nothing speaks against using that function
>>> directly across X pages with a single call.
>>
>> There currently is no point to this, is there?
>> kvm_read/write_guest break the region up into pages anyway,
>> so no reason to try to identify larger continuous chunks.
> 

Right, we're changing the calls from e.g., kvm_write_guest() and
write_guest_abs() to kvm_write_guest_page().

As we're not exposing this function via arch/s390/kvm/gaccess.h, I think
it's ok. Because for external functions we have nice function names like
write_guest_abs(), write_guest_real(), write_guest_lc(), write_guest(),
which implicitly state in their name which kind of address they expect.
access_guest_page() now accepts an absolute address whereby
access_guest() accepts a virtual address. This is for example different
to kvm_read_guest() and kvm_read_guest_page(), which expect absolute
addresses. But there, the _page functions are not internal helpers.

> 
> @David: How strongly do you feel about this?

Not strongly :)

-- 
Thanks,

David / dhildenb