Re: [PATCH v3 1/2] xen+tools: Report Interrupt Controller Virtualization capabilities on x86

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.02.2022 17:02, Jane Malalane wrote:
> On 24/02/2022 14:08, Jan Beulich wrote:
>> On 18.02.2022 18:29, Jane Malalane wrote:
>>> Add XEN_SYSCTL_PHYSCAP_ARCH_ASSISTED_xapic and
>>> XEN_SYSCTL_PHYSCAP_ARCH_ASSISTED_x2apic to report accelerated xapic
>>> and x2apic, on x86 hardware.
>>> No such features are currently implemented on AMD hardware.
>>>
>>> For that purpose, also add an arch-specific "capabilities" parameter
>>> to struct xen_sysctl_physinfo.
>>>
>>> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>> Signed-off-by: Jane Malalane <jane.malalane@xxxxxxxxxx>
>>> ---
>>> v3:
>>>   * Define XEN_SYSCTL_PHYSCAP_ARCH_MAX for ABI checking and actually
>>>     set arch_capbilities, via a call to c_bitmap_to_ocaml_list()
>>>   * Have assisted_x2apic_available only depend on
>>>     cpu_has_vmx_virtualize_x2apic_mode
>>
>> I understand this was the result from previous discussion, but this
>> needs justifying in the description. Not the least because it differs
>> from when XEN_HVM_CPUID_X2APIC_VIRT would be set as well as from what
>> vmx_vlapic_msr_changed() does. The difference between those two is
>> probably intended (judging from a comment there), but the further
>> difference to what you add isn't obvious.
> 
> Okay, I will make that explicit.
> 
>> Which raises another thought: If that hypervisor leaf was part of the
>> HVM feature set, the tool stack could be able to obtain the wanted
>> information without altering sysctl (assuming the conditions to set
>> the respective bits were the same). And I would view it as generally
>> reasonable for there to be a way for tool stacks to know what
>> hypervisor leaves guests are going to get to see (at the maximum and
>> by default).
> 
> Like the "cpuid" xtf test allows us to?

I don't think I understand the question. That xtf test is concerned
about checking the CPUID output it gets to see itself. It doesn't care
about what other guests might get to see, nor the maximum and default.

> Makes sense to me. I'm happy to take that up after.

"After" what?

Jan