Re: [RFC PATCH 21/21] KVM: x86: Ignore splitting huge pages in fault path for TDX

[Yan Zhao]

On Wed, May 14, 2025 at 05:58:41AM +0800, Edgecombe, Rick P wrote:
> On Thu, 2025-04-24 at 11:09 +0800, Yan Zhao wrote:
> 
> >  int tdx_sept_split_private_spt(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > -			       void *private_spt)
> > +			       void *private_spt, bool mmu_lock_shared)
> >  {
> >  	struct page *page = virt_to_page(private_spt);
> >  	int ret;
> > @@ -1842,6 +1842,29 @@ int tdx_sept_split_private_spt(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> >  	if (KVM_BUG_ON(to_kvm_tdx(kvm)->state != TD_STATE_RUNNABLE || level != PG_LEVEL_2M, kvm))
> >  		return -EINVAL;
> >  
> > +	/*
> > +	 * Split request with mmu_lock held for reading can only occur when one
> > +	 * vCPU accepts at 2MB level while another vCPU accepts at 4KB level.
> > +	 * Ignore this 4KB mapping request by setting violation_request_level to
> > +	 * 2MB and returning -EBUSY for retry. Then the next fault at 2MB level
> > +	 * would be a spurious fault. The vCPU accepting at 2MB will accept the
> > +	 * whole 2MB range.
> > +	 */
> > +	if (mmu_lock_shared) {
> > +		struct kvm_vcpu *vcpu = kvm_get_running_vcpu();
> > +		struct vcpu_tdx *tdx = to_tdx(vcpu);
> > +
> > +		if (KVM_BUG_ON(!vcpu, kvm))
> > +			return -EOPNOTSUPP;
> > +
> > +		/* Request to map as 2MB leaf for the whole 2MB range */
> > +		tdx->violation_gfn_start = gfn_round_for_level(gfn, level);
> > +		tdx->violation_gfn_end = tdx->violation_gfn_start + KVM_PAGES_PER_HPAGE(level);
> > +		tdx->violation_request_level = level;
> > +
> > +		return -EBUSY;
> 
> This is too hacky the way it infers so much from mmu_lock_shared. Since guests
> shouldn't be doing this, what about just doing kvm_vm_dead(), with a little
> pr_warn()? Maybe even just do it in set_external_spte_present() and declare it
There's a valid case [1] besides double accept to trigger demotion in the fault
path. Kirill believed we need to support that case [2].

KVM MMU core can't tell if the demotion is caused by double accept or not.

[1] https://lore.kernel.org/all/aAn3SSocw0XvaRye@xxxxxxxxxxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/all/6vdj4mfxlyvypn743klxq5twda66tkugwzljdt275rug2gmwwl@zdziylxpre6y/

> the rule for external page tables. It can shrink this patch significantly, for
> no expected user impact.


> 
> > +	}
> > +
> >  	ret = tdx_sept_zap_private_spte(kvm, gfn, level, page);
> >  	if (ret <= 0)
> >  		return ret;
> > diff --git a/arch/x86/kvm/vmx/x86_ops.h b/arch/x86/kvm/vmx/x86_ops.h
> > index 0619e9390e5d..fcba76887508 100644
> > --- a/arch/x86/kvm/vmx/x86_ops.h
> > +++ b/arch/x86/kvm/vmx/x86_ops.h
> > @@ -159,7 +159,7 @@ int tdx_sept_set_private_spte(struct kvm *kvm, gfn_t gfn,
> >  int tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
> >  				 enum pg_level level, kvm_pfn_t pfn);
> >  int tdx_sept_split_private_spt(struct kvm *kvm, gfn_t gfn, enum pg_level level,
> > -			       void *private_spt);
> > +			       void *private_spt, bool mmu_lock_shared);
> >  
> >  void tdx_flush_tlb_current(struct kvm_vcpu *vcpu);
> >  void tdx_flush_tlb_all(struct kvm_vcpu *vcpu);
> > @@ -228,7 +228,8 @@ static inline int tdx_sept_remove_private_spte(struct kvm *kvm, gfn_t gfn,
> >  
> >  static inline int tdx_sept_split_private_spt(struct kvm *kvm, gfn_t gfn,
> >  					     enum pg_level level,
> > -					     void *private_spt)
> > +					     void *private_spt,
> > +					     bool mmu_lock_shared)
> >  {
> >  	return -EOPNOTSUPP;
> >  }
>