Re: [PATCH v2] VMX: use a single, global APIC access page

[Jan Beulich <jbeulich@suse.com>]

On 19.02.2021 17:45, Jan Beulich wrote:
> The address of this page is used by the CPU only to recognize when to
> access the virtual APIC page instead. No accesses would ever go to this
> page. It only needs to be present in the (CPU) page tables so that
> address translation will produce its address as result for respective
> accesses.
> 
> By making this page global, we also eliminate the need to refcount it,
> or to assign it to any domain in the first place.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> v2: Avoid insertion when !has_vlapic(). Split off change to
>     p2m_get_iommu_flags().
> ---
> Hooking p2m insertion onto arch_domain_creation_finished() isn't very
> nice, but I couldn't find any better hook (nor a good place where to
> introduce a new one). In particular there look to be no hvm_funcs hooks
> being used on a domain-wide basis (except for init/destroy of course).
> I did consider connecting this to the setting of HVM_PARAM_IDENT_PT, but
> considered this no better, the more that the tool stack could be smarter
> and avoid setting that param when not needed.

While this patch was triggered not just by Julien's observation of
the early p2m insertion being a problem, but also many earlier
times of running into this odd code, it is - especially at this
stage - perhaps a possible option to split the change into just
the movement of the set_mmio_p2m_entry() invocation and all the
rest, in order to defer that rest until after 4.15.

Jan