Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

[Dave Hansen <dave.hansen@intel.com>]

On 1/16/26 09:14, Sean Christopherson wrote:
...
>> *EVEN* if the pfn_to_page() itself is unsafe, and even if the WARN()s
>> are compiled out, this explicitly lays out the assumptions and it means
>> someone reading TDX code has an easier idea comprehending it.
> I object to the existence of those assumptions.  Why the blazes does TDX care
> how KVM and guest_memfd manages memory? 

For me, it's because TDX can't take arbitrary kvm_pfn_t's for private
memory. It's got to be able to be converted in the hardware, and also
have allocated TDX metadata (PAMT) that the TDX module was handed at
module init time. I thought kvm_pfn_t might, for instance be pointing
over to a shared page or MMIO. Those can't be used for TD private memory.

I think it's a pretty useful convention to know that the generic,
flexible kvm_pfn_t has been winnowed down to a more restrictive type
that is what TDX needs.

But, honestly, my big aversion was to u64's everywhere. I can certainly
live with a few kvm_pfn_t's in the TDX code. It doesn't have to be
'struct page'.

> If you want to assert that the pfn is compatible with TDX, then by
> all means.  But I am NOT accepting any more KVM code that assumes
> TDX memory is backed by refcounted struct page.  If I had been
> paying more attention when the initial TDX series landed, I would
> have NAK'd that too.
I'm kinda surprised by that. The only memory we support handing into TDs
for private memory is refcounted struct page. I can imagine us being
able to do this with DAX pages in the near future, but those have
'struct page' too, and I think they're refcounted pretty normally now as
well.

The TDX module initialization is pretty tied to NUMA nodes, too. If it's
in a NUMA node, the TDX module is told about it and it also universally
gets a 'struct page'.

Is there some kind of memory that I'm missing? What else *is* there? :)

> tdh_mem_page_aug() is just an absurdly slow way of writing a PTE.  It doesn't
> _need_ the pfn to be backed a struct page, at all.  IMO, what you're asking for
> is akin to adding a pile of unnecessary assumptions to e.g. __set_spte() and
> __kvm_tdp_mmu_write_spte().  No thanks.

Which part is absurdly slow? page_to_phys()? Isn't that just a shift by
an immediate and a subtraction of an immediate? Yeah, the subtraction
immediate is chonky so the instruction is big.

But at a quick glance I'm not seeing anything absurd.