Message-ID: <62fb425d-4bda-63e0-469b-f0ae43539929@redhat.com>
Date:   Mon, 22 Nov 2021 12:13:24 +0100
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To:     Janosch Frank <frankja@linux.ibm.com>,
        Janis Schoetterl-Glausch <scgl@linux.vnet.ibm.com>,
        Janis Schoetterl-Glausch <scgl@linux.ibm.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>
Cc:     Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Alexander Gordeev <agordeev@linux.ibm.com>,
        kvm@vger.kernel.org, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20211028135556.1793063-1-scgl@linux.ibm.com>
 <20211028135556.1793063-4-scgl@linux.ibm.com>
 <4ac7c459-8e13-087a-f98d-9f3e0e6d8ee6@redhat.com>
 <457896b2-b462-639e-bb40-dee3716fcb9a@linux.vnet.ibm.com>
 <1a380055-536e-123d-499e-40314cf35f44@linux.ibm.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Subject: Re: [PATCH v2 3/3] KVM: s390: gaccess: Cleanup access to guest frames
In-Reply-To: <1a380055-536e-123d-499e-40314cf35f44@linux.ibm.com>
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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