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Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page pois
From: |
David Hildenbrand |
Subject: |
Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature |
Date: |
Thu, 16 Apr 2020 16:55:43 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 |
>> We should document our result of page poisoning, free page hinting, and
>> free page reporting there as well. I hope you'll have time for the latter.
>>
>> -------------------------------------------------------------------------
>> Semantics of VIRTIO_BALLOON_F_PAGE_POISON
>> -------------------------------------------------------------------------
>>
>> "The VIRTIO_BALLOON_F_PAGE_POISON feature bit is used to indicate if the
>> guest is using page poisoning. Guest writes to the poison_val config
>> field to tell host about the page poisoning value that is in use."
>> -> Very little information, no signs about what has to be done.
>
> I think it's an informational field. Knowing that free pages
> are full of a specific pattern can be handy for the hypervisor
> for a variety of reasons. E.g. compression/deduplication?
I was referring to the documentation of the feature and what we
(hypervisor) are expected to do (in regards to inflation/deflation).
Yes, it might be valuable to know that the guest is using poisoning. I
assume compression/deduplication (IOW KSM) will figure out themselves
that such pages are equal.
>> "Let the hypervisor know that we are expecting a specific value to be
>> written back in balloon pages."
>
>
>
>> -> Okay, that talks about "balloon pages", which would include right now
>> -- pages "inflated" and then "deflated" using free page hinting
>> -- pages "inflated" and then "deflated" using oridnary inflate/deflate
>> queue
>
> ATM, in this case driver calls "free" and that fills page with the
> poison value.
Yes, that's what I mentioned somehwere, it's currently done by Linux and ...
>
> It might be a valid optimization to allow driver to skip
> poisoning of freed pages in this case.
... we should prepare for that :)
>
>> And I would add
>>
>> "However, if the inflated page was not filled with "poison_val" when
>> inflating, it's not predictable if the original page or a page filled
>> with "poison_val" is returned."
>>
>> Which would cover the "we did not discard the page in the hypervisor, so
>> the original page is still there".
>>
>>
>> We should also document what is expected to happen if "poison_val" is
>> suddenly changed by the guest at one point in time again. (e.g., not
>> supported, unexpected things can happen, etc.)
>
> Right. I think we should require that this can only be changed
> before features have been negotiated.
> That is the only point where hypervisor can still fail
> gracefully (i.e. fail FEATURES_OK).
Agreed.
I can totally understand if Alex would want to stop working on
VIRTIO_BALLOON_F_PAGE_POISON at this point and only fix the guest to not
enable free page reporting in case we don't have
VIRTIO_BALLOON_F_PAGE_POISON (unless that's already done), lol. :)
--
Thanks,
David / dhildenb
- [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, (continued)
- [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/09
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/15
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Michael S. Tsirkin, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature,
David Hildenbrand <=
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, Alexander Duyck, 2020/04/16
- Re: [PATCH v19 QEMU 1/4] virtio-balloon: Implement support for page poison tracking feature, David Hildenbrand, 2020/04/16
[PATCH v19 QEMU 2/4] linux-headers: update to contain virito-balloon free page reporting, Alexander Duyck, 2020/04/09
[PATCH v19 QEMU 3/4] virtio-balloon: Provide an interface for free page reporting, Alexander Duyck, 2020/04/09
[PATCH v19 QEMU 4/4] memory: Do not allow direct write access to rom_device regions, Alexander Duyck, 2020/04/09