Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

[Edgecombe, Rick P]

On Tue, 2024-03-19 at 16:56 -0700, Isaku Yamahata wrote:
> When we zap a page from the guest, and add it again on TDX even with
> the same
> GPA, the page is zeroed.  We'd like to keep memory contents for those
> cases.
> 
> Ok, let me add those whys and drop migration part. Here is the
> updated one.
> 
> TDX supports only write-back(WB) memory type for private memory
> architecturally so that (virtualized) memory type change doesn't make
> sense for private memory.  When we remove the private page from the
> guest
> and re-add it with the same GPA, the page is zeroed.
> 
> Regarding memory type change (mtrr virtualization and lapic page
> mapping change), the current implementation zaps pages, and populate
                                                                     s^
> the page with new memory type on the next KVM page fault.  
                               ^s

> It doesn't work for TDX to have zeroed pages.
What does this mean? Above you mention how all the pages are zeroed. Do
you mean it doesn't work for TDX to zero a running guest's pages. Which
would happen for the operations that would expect the pages could get
faulted in again just fine.


> Because TDX supports only WB, we
> ignore the request for MTRR and lapic page change to not zap private
> pages on unmapping for those two cases

Hmm. I need to go back and look at this again. It's not clear from the
description why it is safe for the host to not zap pages if requested
to. I see why the guest wouldn't want them to be zapped.

> 
> TDX Secure-EPT requires removing the guest pages first and leaf
> Secure-EPT pages in order. It doesn't allow zap a Secure-EPT entry
> that has child pages.  It doesn't work with the current TDP MMU
> zapping logic that zaps the root page table without touching child
> pages.  Instead, zap only leaf SPTEs for KVM mmu that has a shared
> bit
> mask.

Could this be better as two patches that each address a separate thing?
1. Leaf only zapping
2. Don't zap for MTRR, etc.

> > 
> > There seems to be an attempt to abstract away the existence of
> > Secure-
> > EPT in mmu.c, that is not fully successful. In this case the code
> > checks kvm_gfn_shared_mask() to see if it needs to handle the
> > zapping
> > in a way specific needed by S-EPT. It ends up being a little
> > confusing
> > because the actual check is about whether there is a shared bit. It
> > only works because only S-EPT is the only thing that has a
> > kvm_gfn_shared_mask().
> > 
> > Doing something like (kvm->arch.vm_type == KVM_X86_TDX_VM) looks
> > wrong,
> > but is more honest about what we are getting up to here. I'm not
> > sure
> > though, what do you think?
> 
> Right, I attempted and failed in zapping case.  This is due to the
> restriction
> that the Secure-EPT pages must be removed from the leaves.  the VMX
> case (also
> NPT, even SNP) heavily depends on zapping root entry as optimization.
> 
> I can think of
> - add TDX check. Looks wrong
> - Use kvm_gfn_shared_mask(kvm). confusing
> - Give other name for this check like zap_from_leafs (or better
> name?)
>   The implementation is same to kvm_gfn_shared_mask() with comment.
>   - Or we can add a boolean variable to struct kvm

Hmm, maybe wrap it in a function like:
static inline bool kvm_can_only_zap_leafs(const struct kvm *kvm)
{
	/* A comment explaining what is going on */
	return kvm->arch.vm_type == KVM_X86_TDX_VM;
}

But KVM seems to be a bit more on the open coded side when it comes to
things like this, so not sure what maintainers would prefer. My opinion
is the kvm_gfn_shared_mask() check is too strange and it's worth a new
helper. If that is bad, then just open coded kvm->arch.vm_type ==
KVM_X86_TDX_VM is the second best I think.

I feel both strongly that it should be changed, and unsure what
maintainers would prefer. Hopefully one will chime in.