Re: [PATCH v5] x86: make Viridian support optional

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Oct 17, 2025 at 08:01:13AM +0200, Jan Beulich wrote:
> On 17.10.2025 00:38, Marek Marczykowski-Górecki wrote:
> > On Fri, Oct 17, 2025 at 12:40:33AM +0300, Grygorii Strashko wrote:
> >> Another point, assume change like this is to be done for HVM_PARAM_VIRIDIAN
> >> - there are another HVM_PARAM_x which depend on build-time disabled 
> >> features, like:
> >>  HVM_PARAM_VM86_TSS_SIZED
> >>  HVM_PARAM_PAGING_RING_PFN,
> >>  HVM_PARAM_MONITOR_RING_PFN,
> >>  HVM_PARAM_SHARING_RING_PFN,
> >>  HVM_PARAM_IDENT_PT
> >>  ...
> >>
> >> if corresponding features are build-time disabled, above HVM_PARAM_x
> >> become R/W with functionality NOP now.
> > 
> > Are you sure? For me it looks like setting build-time disabled feature
> > returns -ENOSYS. Or do you mean some other interface than
> > xc_hvm_param_set().
> 
> Where do you see that ENOSYS coming from? In fact, for example, I don't see 
> any
> of the *_RING_PFN even mentioned at all in hvm.c's parameter handling. Are you
> perhaps thinking of only the HVM=n case, whereas I expect Grygorii talks about
> the more fine-grained controls?

Oh, sorry, I looked at XEN_DOMCTL_vm_event_op -> vm_event_domctl()...

> That said, whether to uniformly fail requests for params solely related to
> build-time disabled features isn't quite clear. Arguably for e.g. paging and
> sharing, setting the ring PFN can as well be silently ignored (no events ever
> appearing), while failure would then be reported from other compiled-out 
> logic.
> 
> Jan

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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