linux-kernel - Re: [PATCH 2/2] KVM: selftests: Enable dirty_log

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Date:   Wed, 31 Jul 2019 13:06:03 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Thomas Huth <thuth@...hat.com>, kvm@...r.kernel.org,
        Janosch Frank <frankja@...ux.ibm.com>
Cc:     linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-s390@...r.kernel.org, Cornelia Huck <cohuck@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Shuah Khan <shuah@...nel.org>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH 2/2] KVM: selftests: Enable dirty_log_test on s390x

On 30.07.19 20:04, Christian Borntraeger wrote:
> 
> 
> On 30.07.19 19:11, Thomas Huth wrote:
>> On 30/07/2019 16.57, Christian Borntraeger wrote:
>>>
>>>
>>> On 30.07.19 12:01, Thomas Huth wrote:
>>>> To run the dirty_log_test on s390x, we have to make sure that we
>>>> access the dirty log bitmap with little endian byte ordering and
>>>> we have to properly align the memslot of the guest.
>>>> Also all dirty bits of a segment are set once on s390x when one
>>>> of the pages of a segment are written to for the first time, so
>>>> we have to make sure that we touch all pages during the first
>>>> iteration to keep the test in sync here.
>>>
>>> While this fixes the test (and the migration does work fine), it still
>>> means that s390x overindicates the dirty bit for sparsely populated
>>> 1M segments. It is just a performance issue, but maybe we should try 
>>> to get this fixed.
>>
>> I hope you don't expect me to fix this - the gmap code is really not my
>> turf...
> 
> No, this is clearly on our turf. 

FWIW, we share the pagetables with the userspace process. We mark a page
as dirty (PGSTE_UC_BIT) when
- We modify the storage key
- We map a PTE as RW (pgste_set_pte())

I assume all PTEs of the segment are mapped RW (for example, if user
space wrote to such a PTE), that is why we have the PGSTE_UC_BIT bit set.

As PGSTE_UC_BIT also tracks what userspace did, not only KVM via the
GMAP, this might indeed be correct.

-- 

Thanks,

David / dhildenb