Re: ABI coupling to hypervisors via CONFIG_PARAVIRT

[Ingo Molnar]

* Chris Wright <chrisw@xxxxxxxxxxxx> wrote:

> * Ingo Molnar (mingo@xxxxxxx) wrote:
> > ( if there is no backwards compatibility promise then i have zero
> >   complaints: then paravirt_ops + the hypercall just becomes another API
> >   internal to Linux that we can improve at will. But that is not
> >   realistic: if we provide CONFIG_VMI today, people will expect to have
> >   CONFIG_VMI in the future too. )
> 
> This was the whole reason we didn't adopt VMI directly.  Instead, 
> preferring an kernel internal API, pv_ops, that can adopt naturally as 
> the kernel changes, and it is the pv_ops client code's (or backend as 
> it is also referred to) responsibility to do whatever is necessary to 
> map back to the hypervisor's ABI.  The goal was explicitly to keep 
> things internal fluid as usual.  As I said before, no matter how you 
> slice it there's glue code somewhere to deal with compatibilities. And 
> it's always been the virtualization platform's responsibility to deal 
> with the changes.

For example, for the sake of argument, if the VMI ABI consisted only of 
a single call:

  #define VMI_CALL_NOP           1

then obviously it would be very hard for VMI to adopt to changes in the 
kernel - no matter how many smarts you put into paravirt_ops :-)

agreed? That is the center of my argument. Does the VMI ABI limit the 
Linux kernel or not?

As we increase the complexity of a hypercall ABI, more and more things 
can be implemented via it. So _obviously_ there is a 'minimum level of 
capability' for every hypercall ABI that is /required/ to keep the Linux 
kernel 100% flexible. If in some tricky corner the ABI has some stupid 
limit or assumption, it might stiffle future changes in Linux.

i am worried whether /any/ future change to the upstream kernel's design 
can be adopted via paravirt_ops, via the current VMI ABI. And by /any/ i 
mean truly any. And whether that can be done is not a function of the 
flexibility of paravirt_ops, it's a function of the flexibility of the 
VMI ABI.

	Ingo
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