Re: [PATCH 0/4] KVM: x86: Document and enforce APIC base memory hole

[Sean Christopherson <seanjc@google.com>]

On Mon, Jul 06, 2026, Tim Wiederhake wrote:
> When an in-kernel irqchip is enabled on x86, KVM installs a private
> memory slot at the default APIC base address (0xfee00000) during vcpu
> creation.  If user space has already mapped a memory region covering
> that address, vcpu creation fails with -EEXIST.  The same happens in
> reverse: mapping memory over the APIC base after vcpu creation also
> fails with -EEXIST.
> 
> None of this is documented, and the error is reported far from where
> the actual conflict is introduced.  A VMM developer hitting this has
> to trace through KVM internals to understand what went wrong.
> 
> This series documents the two undocumented constraints (irqchip before
> vcpu, APIC base memory hole) and adds early checks so the error is
> reported at the ioctl that actually violates the constraint.
> 
> Patches 1-2 are documentation only.  Patches 3-4 add early validation
> that turns a confusing -EEXIST at vcpu creation into an explicit error
> at the point where the conflict is introduced.

I'm 100% in favor of documenting the behavior, but I'm not exactly excited about
the enforcement.  It's not needed for KVM's safety, and while I appreciate that
such a goof would be somewhat annoying to debug, I have a hard time believing it's
a common failure.  I.e. I'm not convinced carrying the code is worth the marginal
benefits it provides.