Re: [PATCH v4 1/2] xen+tools: Report Interrupt Controller Virtualization capabilities on x86

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.03.2022 13:17, Jane Malalane wrote:
> On 04/03/2022 08:17, Jan Beulich wrote:
>> [CAUTION - EXTERNAL EMAIL] DO NOT reply, click links, or open attachments 
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>> On 03.03.2022 17:37, Jane Malalane wrote:
>>> On 03/03/2022 11:37, Jan Beulich wrote:
>>>> On 02.03.2022 16:00, 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.
>>>>>
>>>>> Note that this interface is intended to be compatible with AMD so that
>>>>> AVIC support can be introduced in a future patch. Unlike Intel that
>>>>> has multiple controls for APIC Virtualization, AMD has one global
>>>>> 'AVIC Enable' control bit, so fine-graining of APIC virtualization
>>>>> control cannot be done on a common interface. Therefore, for xAPIC HW
>>>>> assisted virtualization support to be reported, HW must support
>>>>> virtualize_apic_accesses as well as apic_reg_virt.
>>>>
>>>> Okay, here you now describe _what_ is being implemented, but I'm
>>>> afraid it still lacks justification (beyond making this re-usable for
>>>> AVIC, which imo can only be a secondary goal). You actually say ...
> Is the following any better...?
> 
> "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.
> 
> HW assisted xAPIC virtualization will be reported if HW, at the minimum, 
>   supports virtualize_apic_accesses as this feature alone means that an 
> access to the APIC page will cause an APIC-access VM exit. An 
> APIC-access VM exit provides a VMM with information about the access 
> causing the VM exit, unlike a regular EPT fault, thus simplifying some 
> internal handling.
> 
> HW assisted x2APIC virtualization will be reported if HW supports 
> virtualize_x2apic_mode and, at least, either apic_reg_virt or 
> virtual_intr_delivery. This is due to apic_reg_virt and 
> virtual_intr_delivery preventing a VM exit from occuring or at least 
> replacing a regular EPT fault VM-exit with an APIC-access VM-exit on 
> read and write APIC accesses, respectively.
> This also means that sysctl follows the conditionals in 
> vmx_vlapic_msr_changed().
> 
> For that purpose, also add an arch-specific "capabilities" parameter
> to struct xen_sysctl_physinfo.
> 
> Note that this interface is intended to be compatible with AMD so that
> AVIC support can be introduced in a future patch. Unlike Intel that
> has multiple controls for APIC Virtualization, AMD has one global
> 'AVIC Enable' control bit, so fine-graining of APIC virtualization
> control cannot be done on a common interface."

Yes, this looks quite a bit better, thanks. Assuming, of course, it's
in sync with the code in v5 ...

Jan