Re: [PATCH v4 08/15] KVM: s390: interfaces to (de)configure guest's AP matrix

[Tony Krowiak]

On 05/03/2018 12:01 PM, Pierre Morel wrote:
> On 03/05/2018 16:41, Tony Krowiak wrote:
> > On 05/02/2018 10:57 AM, Pierre Morel wrote:
> > > On 25/04/2018 18:21, Tony Krowiak wrote:
> > > > On 04/23/2018 09:46 AM, Pierre Morel wrote:
> > > > > On 15/04/2018 23:22, Tony Krowiak wrote:
> > > > > > Provides interfaces to assign AP adapters, usage domains
> > > > > > and control domains to a KVM guest.
> ...
> > The kvm_ap_configure_matrix(kvm, matrix) interface configures the 
> > APM, AQM and ADM in the
> > guest's CRYCB which implies AP instructions are being interpreted. 
> > There is nothing in SIE
> > precluding the sharing of AP queues between guests using SIE to 
> > interpret AP instructions,
> > it is my opinion - along with several others - that this is not 
> > advisable given the
> > results are not predictable, not to mention the security concerns. If 
> > the protocol to access
> > queues changes, then we create a different interface. The other 
> > option is to include a flag
> > on the kvm_ap_configure_matrix(kvm, matrix) interface to indicate 
> > whether sharing is
> > allowed. I don't like this, because we have no way of knowing if the 
> > caller has taken the
> > proper care to ensure the VM sharing the queue should be allowed 
> > access. Besides, when
> > queue sharing is implemented, it is my opinion that we will intercept 
> > the AP instructions
> > and the matrix will not be configured in the CRYCB. I stick by my 
> > response above.
>
> I mean, validating the queue sharing is a mater of the VFIO driver.
> This code is not needed if the VFIO driver is not used.
> But it is not very important.

Yes, this check could have been implemented in the VFIO driver, but as I 
stated above, that
would require the driver to "know" the internals of KVM. I think the KVM 
logic should
be encapsulated in KVM. If we want to allow sharing of interpreted AP 
devices, then we
can always add a flag to the interface.

> > > > > > +static int kvm_ap_matrix_apm_create(struct kvm_ap_matrix 
> > > > > > *ap_matrix,
> > > > > > +                    struct ap_config_info *config)
> > > > > > +{
> > > > > > +    int apm_max = (config && config->apxa) ? config->Na + 1 : 16;
> > > > >
> > > > > At this moment you already know the format of the crycb.
> > > >
> > > > How?
> > >
> > > you calculated this in kvm_ap_build_crycbd() which is called from 
> > > kvm_s390_crypto_init()
> > > itself called from kvm_arch_init_vm().
> > > It is when starting the VM.
> >
> > This structure is used by the vfio_ap driver to store the mediated 
> > matrix device's matrix
> > configuration as well as to configure the CRYCB. The mediated 
> > device's matrix is
> > configured before the guest is started ... it is the means for 
> > configuring the guest's
> > matrix after all. The bottom line is, this function will be called 
> > long before the
> > kvm_ap_build_crycbd() function is called.
>
> you are right, I was thinking about open, should have take more 
> attention.
> Sorry.
>
> Pierre