Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2025-05-16 at 17:43 +0800, Zhao, Yan Y wrote:
> On Fri, May 16, 2025 at 09:35:37AM +0800, Huang, Kai wrote:
> > On Tue, 2025-05-13 at 20:10 +0000, Edgecombe, Rick P wrote:
> > > > @@ -3265,7 +3263,7 @@ int tdx_gmem_private_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn)
> > > >   	if (unlikely(to_kvm_tdx(kvm)->state != TD_STATE_RUNNABLE))
> > > >   		return PG_LEVEL_4K;
> > > >   
> > > > -	return PG_LEVEL_4K;
> > > > +	return PG_LEVEL_2M;
> > > 
> > > Maybe combine this with patch 4, or split them into sensible categories.
> > 
> > How about merge with patch 12
> > 
> >   [RFC PATCH 12/21] KVM: TDX: Determine max mapping level according to vCPU's 
> >   ACCEPT level
> > 
> > instead?
> > 
> > Per patch 12, the fault due to TDH.MEM.PAGE.ACCPT contains fault level info, so
> > KVM should just return that.  But seems we are still returning PG_LEVEL_2M if no
> > such info is provided (IIUC):
> Yes, if without such info (tdx->violation_request_level), we always return
> PG_LEVEL_2M.
> 
> 
> > int tdx_gmem_private_max_mapping_level(struct kvm_vcpu *vcpu, kvm_pfn_t pfn, 
> > 				       gfn_t gfn)
> >  {
> > +	struct vcpu_tdx *tdx = to_tdx(vcpu);
> > +
> >  	if (unlikely(to_kvm_tdx(vcpu->kvm)->state != TD_STATE_RUNNABLE))
> >  		return PG_LEVEL_4K;
> >  
> > +	if (gfn >= tdx->violation_gfn_start && gfn < tdx->violation_gfn_end)
> > +		return tdx->violation_request_level;
> > +
> >  	return PG_LEVEL_2M;
> >  }
> > 
> > So why not returning PT_LEVEL_4K at the end?
> > 
> > I am asking because below text mentioned in the coverletter:
> > 
> >     A rare case that could lead to splitting in the fault path is when a TD
> >     is configured to receive #VE and accesses memory before the ACCEPT
> >     operation. By the time a vCPU accesses a private GFN, due to the lack
> >     of any guest preferred level, KVM could create a mapping at 2MB level.
> >     If the TD then only performs the ACCEPT operation at 4KB level,
> >     splitting in the fault path will be triggered. However, this is not
> >     regarded as a typical use case, as usually TD always accepts pages in
> >     the order from 1GB->2MB->4KB. The worst outcome to ignore the resulting
> >     splitting request is an endless EPT violation. This would not happen
> >     for a Linux guest, which does not expect any #VE.
> > 
> > Changing to return PT_LEVEL_4K should avoid this problem.  It doesn't hurt
> For TDs expect #VE, guests access private memory before accept it.
> In that case, upon KVM receives EPT violation, there's no expected level from
> the TDX module. Returning PT_LEVEL_4K at the end basically disables huge pages
> for those TDs.

Just want to make sure I understand correctly:

Linux TDs always ACCEPT memory first before touching that memory, therefore KVM
should always be able to get the accept level for Linux TDs.

In other words, returning PG_LEVEL_4K doesn't impact establishing large page
mapping for Linux TDs.

However, other TDs may choose to touch memory first to receive #VE and then
accept that memory.  Returning PG_LEVEL_2M allows those TDs to use large page
mappings in SEPT.  Otherwise, returning PG_LEVEL_4K essentially disables large
page for them (since we don't support PROMOTE for now?).

But in the above text you mentioned that, if doing so, because we choose to
ignore splitting request on read, returning 2M could result in *endless* EPT
violation.

So to me it seems you choose a design that could bring performance gain for
certain non-Linux TDs when they follow a certain behaviour but otherwise could
result in endless EPT violation in KVM.

I am not sure how is this OK?  Or probably I have misunderstanding?

> 
> Besides, according to Kirill [1], the order from 1GB->2MB->4KB is only the case
> for linux guests.
> 
> [1] https://lore.kernel.org/all/6vdj4mfxlyvypn743klxq5twda66tkugwzljdt275rug2gmwwl@zdziylxpre6y/#t

I am not sure how is this related?

On the opposite, if other non-Linux TDs don't follow 1G->2M->4K accept order,
e.g., they always accept 4K, there could be *endless EPT violation* if I
understand your words correctly.

Isn't this yet-another reason we should choose to return PG_LEVEL_4K instead of
2M if no accept level is provided in the fault?

> 
> > normal cases either, since guest will always do ACCEPT (which contains the
> > accepting level) before accessing the memory.