Re: [PATCH v9 12/22] s390: vfio-ap: sysfs interfaces to configure control domains

[Tony Krowiak]

On 08/22/2018 01:11 PM, Halil Pasic wrote:
> On 08/22/2018 05:48 PM, Christian Borntraeger wrote:
> > On 08/22/2018 05:34 PM, Pierre Morel wrote:
> > > On 22/08/2018 17:11, Christian Borntraeger wrote:
> > > > On 08/22/2018 01:03 PM, Pierre Morel wrote:
> > > > > > That's interesting.
> > > > > >
> > > > > > > IMHO this quote is quite a half-full half-empty cup one:
> > > > > > > * it mandates the set of usage domains is a subset of the set
> > > > > > > of the control domains, but
> > > > > > > * it speaks of independent controls, namely about the 'usage 
> > > > > > > domain index'
> > > > > > > and the 'control domain index list' and makes the enforcement of 
> > > > > > > the rule
> > > > > > > a job of the administrator (instead of codifying it in the 
> > > > > > > controls).
> > > > > > I'm wondering if a configuration with a usage domain that is not 
> > > > > > also a
> > > > > > control domain is rejected outright? Anybody tried that? :)
> > > > >
> > > > > Yes, and no it is not.
> > > > > We can use a queue (usage domain) to a AP card for SHA-512 or RSA 
> > > > > without
> > > > > having to define the queue as a control domain.
> > > >
> > > > Huh? My HMC allows to add a domain as
> > > > - control only domain
> > > > - control and usage domain.
> > > >
> > > > But I am not able to configure a usage-only domain for my LPAR. 
> > > > That seems to match
> > > > the current code, no?
> > >
> > > Yes, it may not be configurable by the HMC but if we start a guest 
> > > with no control domain it is not a problem to access the hardware 
> > > through the usage domain.
> > >
> > > I tested this a long time ago, but tested again today to be sure on 
> > > my LPAR.
> > >
> > > AFAIU adding a control only domain and a control and usage domain
> > > allows say:
> > > control and usage domain 1
> > > control only domain 2
> > >
> > > Allow to send a message to domain 2 using queue 1
> > >
> > > Allow also to send a domain modifying message to domain 1 using queue 1
> > >
> > > control domain are domain which are controlled
> >
> > So you have changed the code to not automatically make a usage domain a
> > control domain in the bitfield (and you could still use it as a usage
> > domain). Correct?
>
> I tested basically the same yesterday, with the same results.
>
> > I think this is probably expected. the "usage implies control" seems to
> > be a convention implemented by HMC (lpar) and z/VM but millicode offers
> > the bits to have usage-only domains. As LPAR and z/VM will always enable
> > any usage-domain to also be a control domain we should do the same.
>
> I'm fine either way, but slightly prefer higher level management software
> and not the kernel accommodating this convention.
>
> Please consider a quote from Harald's mail in another sub-thread
>
>
> """
> ... about control domains
>
> Talked with the s390 firmware guys. The convention that the control 
> domain
> mask is a superset of the usage domain mask is only true for 1st level 
> guests.
>
> It is absolutely valid to run a kvm guest with restricted control domain
> mask bitmap in the CRYCB. It is valid to have an empty control domain 
> mask
> and the guest should be able to run crypto CPRBs on the usage 
> domain(s) without
> any problems. However, nobody has tried this.
> """
>
> I'm yet to get an explanation why was this convention established in 
> the first
> place. And I can not figure it out myself. For me a setup where I know 
> that
> the domains used by some guest can not be modified by the same guest 
> makes
> perfect sense. If I try to think in analogies, I kind of compare 
> modification
> (that is control domain) with write access, and usage (that is usage 
> domain)
> with read access to, let's say a regular file. For me, all options 
> (rw, r, and w)
> do make sense, and if I had to pick the one that makes the least sense 
> I would
> pick write only. The convention is in these terms making read-only 
> illegal. But
> should 'usage only domains' ever get identified as something somebody 
> wants to do
> we can just add an attribute for that. So I'm fine either way.

One of the things I suggested in a private conversation with Christian 
earlier
today was to provide an additional rw sysfs attribute - a boolean - that 
indicates
whether all usage domains should also be control domains. The default 
could be
true. This would allow one to configure guests with usage-only domains 
as well
as satisfy the convention.

> Still I find the commit message for this patch, the implementation of
> assign_control_domain() and also the documentation slightly misleading 
> regarding
> what does one get from assign_domain.



> Regards,
> Halil
>
>
>
>
>
> > > It seems that the HMC enforce the LPARs to have access to their 
> > > usage domain (AFAIU from Harald)