Re: [PATCH v2 2/2] x86/xen: Allow per-domain usage of hardware virtualized APIC

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 07, 2022 at 06:21:01PM +0000, 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 vitualization, 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 running APIC
> read/write accesses without taking a VM exit.
> 
> Being able to disable x{2}APIC hardware assisted vitualization can be
> useful for testing and debugging purposes.

Might be worth adding a note to the commit log in order to note that
vmx_install_vlapic_mapping doesn't require modifications regardless of
whether the guest has virtualize_apic_accesses enabled or not.

Setting the APIC_ACCESS_ADDR VMCS field is fine even if
virtualize_apic_accesses is not enabled for the guest: as long as the
feature is supported by the CPU the field will exist.

Thanks, Roger.