Re: [RFC PATCH 14/21] KVM: x86/tdp_mmu: Invoke split_external_spt hook with exclusive mmu_lock

[Edgecombe, Rick P]

On Mon, 2025-05-19 at 12:01 +0800, Yan Zhao wrote:
> On Sat, May 17, 2025 at 06:11:59AM +0800, Edgecombe, Rick P wrote:
> > On Fri, 2025-05-16 at 17:17 +0800, Yan Zhao wrote:
> > > > Shouldn't this BUG_ON be handled in the split_external_spt implementation? I
> > > > don't think we need another one.
> > > Ok. But kvm_x86_split_external_spt() is not for TDX only.
> > > Is it good for KVM MMU core to rely on each implementation to trigger BUG_ON?
> > 
> > It effectively is for TDX only. At least for the foreseeable future. The naming
> > basically means that people don't have to see "TDX" everywhere when they look in
> > the MMU code.
> Hmm, another reason to add the BUG_ON is to align it with remove_external_spte().
> There's also a KVM_BUG_ON() following the remove_external_spte hook.
> 
> I interpret this as error handling in the KVM MMU core, which returns "void",
> so issuing BUG_ON if ret != 0.

This is related to the other thread about how to handle demote failure. Let's
continue there.

But in general, the amount of KVM_BUG_ON()s we have for mirror EPT is a bit of a
code smell. It's not exclusive to this series. But I'd love if we could keep it
from getting worse.