Messages in this thread | | | Subject | Re: [PATCH v6 21/21] s390: doc: detailed specifications for AP virtualization | From | Tony Krowiak <> | Date | Tue, 3 Jul 2018 11:25:16 -0400 |
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On 07/03/2018 10:30 AM, Cornelia Huck wrote: > On Tue, 3 Jul 2018 15:58:37 +0200 > Halil Pasic <pasic@linux.ibm.com> wrote: > >> On 07/03/2018 03:25 PM, Cornelia Huck wrote: >>> On Tue, 3 Jul 2018 14:20:11 +0200 >>> Halil Pasic <pasic@linux.ibm.com> wrote: >>> >>>> On 07/03/2018 01:52 PM, Cornelia Huck wrote: >>>>> On Tue, 3 Jul 2018 11:22:10 +0200 >>>>> Halil Pasic <pasic@linux.ibm.com> wrote: >>>>> >>>> [..] >>>>>> Let me try to invoke the DASD analogy. If one for some reason wants to detach >>>>>> a DASD the procedure to follow seems to be (see >>>>>> https://www.ibm.com/support/knowledgecenter/en/linuxonibm/com.ibm.linux.z.lgdd/lgdd_t_dasd_online.html) >>>>>> the following: >>>>>> 1) Unmount. >>>>>> 2) Offline possibly using safe_offline. >>>>>> 3) Detach. >>>>>> >>>>>> Detaching a disk that is currently doing I/O asks for trouble, so the admin is encouraged >>>>>> to make sure there is no pending I/O. >>>>> I don't think we can use dasd (block devices) as a good analogy for >>>>> every kind of device (for starters, consider network devices). >>>>> >>>> I did not use it for every kind of device. I used it for AP. I'm >>>> under the impression you find the analogy inappropriate. If, could >>>> you please explain why? >>> I don't think block devices (which are designed to be more or less >>> permanently accessed, e.g. by mounting a file system) have the same >>> semantics as ap devices (which exist as a backend for crypto requests). >>> Not everything that makes sense for a block device makes sense for >>> other devices as well, and I don't think it makes sense here. >>> >> I'm still confused. If it's about frequency of access (as hinted >> by block devices accessed more or less permanently) I'm not sure >> there is a substantial difference. I guess there are scenarios where >> the AP domain is used very seldom (e.g. protected keys --> most of >> the crypto ops done by CPACF but AP unwraps at the beginning), but >> there are such scenarios for block too. >> >> If it's about (persistent) state, I guess it again depends on the >> scenario and on the type of the card. But I may be wrong. > So, let's turn this around: Why do you think that dasd (and not qeth or > whatever) is a good model for ap device unbinding? Because I really > fail to get it... maybe the ap driver maintainers can chime in. > >>>> >>>>>> In case of AP you can interpret my 'in use' as the queue is not empty. In my understanding >>>>>> unbind is supposed to be hard (I used the word radical). That's why I compared it to pulling >>>>>> a cable. So that's why I ask is there stuff the admin is supposed to do before doing the >>>>>> unbind. >>>>> Are you asking for a kind of 'quiescing' operation? I would hope that >>>>> the crypto drivers already can deal with that via flushing the queue, >>>>> not allowing new requests, or whatever. This is not the block device >>>>> case. >>>>> >>>> The current implementation of vfio-ap which is a crypto driver too certainly >>>> can not deal 'with that'. Whether the rest of the drivers can, I don't >>>> know. Maybe Tony can tell. >>> If the current implementation of vfio-ap cannot deal with it (by >>> cleaning up, blocking, etc.), it needs at the very least be documented >>> so that it can be implemented later. I do not know what the SIE will or >>> won't do to assist here (e.g., if you're removing it from some masks, >>> the device will already be inaccessible to the guest). But the part you >>> were referring to was talking about the existing host driver anyway, >>> wasn't it? >>> >> I was thinking about both directions. Re-classifying a device form >> pass-through to normal should also be possible. But the document only >> talks about one direction. > Presumably because it (rightfully) focuses on setting up vfio-ap?
This is a true statement. The doc is not intended to be a comprehensive administration guide, it is intended to be more of a design spec. The goal here is to show the relationship between the objects involved in AP queue pass-through.
> >> I'm not familiar with the existing host drivers. If we can say 'Hey, >> unbind is perfectly safe at any time: no per-cautions need to be considered!' >> I'm very happy with that. Although I would find it a bit surprising. >> >> I just wanted to make sure this is not something we forget. >> >>>> I'm aware of the fact that AP adapters are not block devices. But >>>> as stated above I don't understand what is the big difference regarding >>>> the unbind operation. >>>> >>>>> Anyway, this is an administrative issue. If you don't have a clear >>>>> concept which devices are for host usage and which for guest usage, you >>>>> already have problems. >>>> I'm trying to understand the whole solution. I agree, this is an administrative >>>> issue. But the document is trying to address such administrative issues. >>> I'd assume "know which devices are for the host and which devices are >>> for the guests" to be a given, no? >>> >> My other email scratches this topic. AFAIK we don't have a solution for >> that yet. Nor we have a good understanding of how and to what extent >> is statically given what is given. E.g. if one wants to re-partition my AP >> resources (and at some point one will have to at least do the initial >> re-partitioning) do I need a reboot for the changes to take effect? Or >> is this 'known' variable during the uptime of an OS. > I think that is really out of scope for this file, which I'd expect to > explain how vfio-ap basically works and which incantations I need to > give crypto devices to a guest. It should NOT focus on administrative > tasks; this should either be delegated to the likes of libvirt or > documented in a "how to use crypto cards with kvm" kind of technical > writeup. If there's a limitation (e.g. you can't easily unbind again), > write a line here.
On this we can agree.
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