Re: [PATCH v8 23/46] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION

[Sean Christopherson <seanjc@google.com>]

On Tue, Jun 30, 2026, Yan Zhao wrote:
> On Tue, Jun 30, 2026 at 08:35:49AM +0800, Sean Christopherson wrote:
> > Gah, I thought I had sent this out this morning, long before Ackerley's response.
> > But I got distracted by a meeting and forgot to get back to this... *sigh*
> > 
> > Sending what I already wrote, even though there's a lot of overlap with Ackerley's
> > mail.
> > 
> > On Mon, Jun 29, 2026, Yan Zhao wrote:
> > > On Fri, Jun 26, 2026 at 08:28:32AM -0700, Ackerley Tng wrote:
> > > > Yan Zhao <yan.y.zhao@intel.com> writes:
> > > > > But if a user configures 0 uaddr as valid, writes to it, and then passes 0 as
> > > > > source_addr(not from gmem), I'm not sure if it's good for the kernel to silently
> > > > > treat 0 uaddr as an identifier for in-place copy from the private PFN in gmem.
> > > > >
> > > > 
> > > > I'd say the original uAPI perhaps just didn't document 0 as an
> > > > unsupported uaddr. Given that commit 2a62345b3052 already merged, uAPI
> > > > was perhaps accidentally changed and no customer complained, I think we
> > > > can move forward with 0 as an invalid src_address? I wouldn't think
> > > > anyone relies on 0 intentionally being a valid address.
> > > > 
> > > > I could document that, if it helps?
> > > What about just documenting that 0 is an unsupported uaddr which will be
> > > re-purposed as an indicator to use the target pfn as the source, regardless of
> > > whether gmem_in_place_conversion is true? i.e.,
> > > 
> > > if (!src_page) 
> > > 	src_page = pfn_to_page(pfn);
> > 
> > Because KVM can't generally use the target page as the source without in-place
> > conversion, it's not supported today, and out-of-place conversion is being
> > deprecated.
> By "out-of-place conversion", do you mean using per-VM memory attribute
> conversion?

Yep, I couldn't come up with a better description.

> > > I don't get why the two scenarios should be treated differently:
> > > 1. gmem_in_place_conversion==true, shared memory is not from gmem 
> > > 2. gmem_in_place_conversion==false, shared memory is not from gmem
> > > 
> > > In both case, a 0 uaddr could be mapped to a valid page not from gmem.
> > 
> > That's immaterial.  KVM's ABI (that we're solidifying) is that an address of '0'
> > for the source means NULL.  The fact that userspace could have a valid mapping
> > at virtual address '0' is irrelevant.
> So, I'm wondering if we can document that 0 uaddr could always mean using target
> PFN.

I would document it as saying "no source page", and then state that a source page
is required if in-place conversion isn't enabled/supported/allowed.

> i.e., for both scenarios 1 and 2, al long as 0 uaddr is specified, we always
> use target PFN as source for in-place add.
> 
> > Again, just because something is technically possible doesn't mean it needs to
> > be supported by every piece of KVM's uAPI.
> > 
> > > So why not update the uAPI to handle both cases consistently? :)
> > 
> > Because retroactively adding support for out-of-place conversion is pointless
> > (requires a userspace update for a feature that's being deprecated), KVM can't
> > generally support using the source for out-of-place conversion (it's effectively
> > an obscure zero-page optimization), and IMO rejecting the out-of-place conversion
> > scenario is valuable for KVM developers, e.g. to help newcomers understand what
> > exactly is and isn't possible.
> Ok. You mean per-VM memory attribute is deprecating, and source page from !gmem
> backend is also deprecating, so we don't want to change uAPI for scenarios under
> gmem_in_place_conversion==false. Right?

Right.

> 
> > Side topic, isn't TDX broken if target page has already been added to the TD?
> > IIUC, kvm_tdp_mmu_map_private_pfn() will be a glorified nop due to the page
> > already having a valid S-EPT mapping, and so KVM will incorrectly allow a double
> Not sure if my understand out-of-place conversion correctly.
> Given target PFNs and GFNs are not duplicated, what would cause double add? :)

I was working through what would happen if userspace did KVM_TDX_INIT_MEM_REGION
on the same target page multiple times.

> 
> > add.  Ahhh, no, because KVM will return RET_PF_SPURIOUS and
> > kvm_tdp_mmu_map_private_pfn() will then return -EIO.
> My asking was if we could document uaddr always means using target PFN, since
> TDX's in-place add does not rely on gmem in-place conversion.

Yeah, I was on a tangent, ignore everything from "Side topic" on.