Re: [PATCH 1/2] KVM: x86/mmu: Add RET_PF_RETRY_INVALID_SLOT for fault retry on invalid slot

[Edgecombe, Rick P]

On Mon, 2025-05-19 at 06:33 -0700, Sean Christopherson wrote:
> Was this hit by a real VMM?  If so, why is a TDX VMM removing a memslot without
> kicking vCPUs out of KVM?
> 
> Regardless, I would prefer not to add a new RET_PF_* flag for this.  At a glance,
> KVM can simply drop and reacquire SRCU in the relevant paths.

During the initial debugging and kicking around stage, this is the first
direction we looked. But kvm_gmem_populate() doesn't have scru locked, so then
kvm_tdp_map_page() tries to unlock without it being held. (although that version
didn't check r == RET_PF_RETRY like you had). Yan had the following concerns and
came up with the version in this series, which we held review on for the list:

> However, upon further consideration, I am reluctant to implement this fix for
> the following reasons:
> - kvm_gmem_populate() already holds the kvm->slots_lock.
> - While retrying with srcu unlock and lock can workaround the
>   KVM_MEMSLOT_INVALID deadlock, it results in each kvm_vcpu_pre_fault_memory()
>   and tdx_handle_ept_violation() faulting with different memslot layouts.


I'm not sure why the second one is really a problem. For the first one I think
that path could just take the scru lock in the proper order with kvm-
>slots_lock? I need to stare at these locking rules each time, so low quality
suggestion. But that is the context.