Re: [PATCH v9 2/2] x86/xen: Allow per-domain usage of hardware virtualized APIC

[Jan Beulich <jbeulich@xxxxxxxx>]

On 01.04.2022 12:47, Jane Malalane wrote:
> Introduce a new per-domain creation x86 specific flag to
> select whether hardware assisted virtualization should be used for
> x{2}APIC.
> 
> A per-domain option is added to xl in order to select the usage of
> x{2}APIC hardware assisted virtualization, as well as a global
> configuration option.
> 
> Having all APIC interaction exit to Xen for emulation is slow and can
> induce much overhead. Hardware can speed up x{2}APIC by decoding the
> APIC access and providing a VM exit with a more specific exit reason
> than a regular EPT fault or by altogether avoiding a VM exit.
> 
> On the other hand, being able to disable x{2}APIC hardware assisted
> virtualization can be useful for testing and debugging purposes.
> 
> Note:
> 
> - vmx_install_vlapic_mapping doesn't require modifications regardless
> of whether the guest has "Virtualize APIC accesses" enabled or not,
> i.e., setting the APIC_ACCESS_ADDR VMCS field is fine so long as
> virtualize_apic_accesses is supported by the CPU.
> 
> - Both per-domain and global assisted_x{2}apic options are not part of
> the migration stream, unless explicitly set in the configuration file,
> so it is safe to migrate a guest that doesn't have assisted_x{2}apic
> set in its config file between hosts that have different support for
> hardware assisted x{2}APIC virtualization.

While I can agree this is the case as far as your changes are concerned,
it reads as if this was a broader statement. Since hypervisor CPUID leaf
contents can change across such a migration (which is pre-existing
behavior), I'm not convinced the statement is generally applicable.

> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Signed-off-by: Jane Malalane <jane.malalane@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx> # hypervisor

Jan