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

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Message-ID: <aW3G6yZuvclYABzP@yzhao56-desk.sh.intel.com>
Date: Mon, 19 Jan 2026 13:53:47 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "seanjc@...gle.com" <seanjc@...gle.com>, "Du, Fan" <fan.du@...el.com>,
	"Li, Xiaoyao" <xiaoyao.li@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
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Subject: Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

On Sat, Jan 17, 2026 at 12:58:02AM +0800, Edgecombe, Rick P wrote:
> On Fri, 2026-01-16 at 08:31 -0800, Sean Christopherson wrote:
> > > Dave wants safety for the TDX pages getting handed to the module.
> > 
> > Define "safety".  As I stressed earlier, blinding retrieving a
> > "struct page" and dereferencing that pointer is the exact opposite of
> > safe.
> 
> I think we had two problems.
> 
> 1. Passing in raw PA's via u64 led to buggy code. IIRC we had a bug
> with this that was caught before it went upstream. So a page needs a
> real type of some sort.
> 
> 2. Work was done on the tip side to prevent non-TDX capable memory from
> entering the page allocator. With that in place, by requiring struct
> page, TDX code can know that it is getting the type of memory it worked
> hard to guarantee was good.
> 
> You are saying that shifting a PFN to a struct page blindly doesn't
> actually guarantee that it meets those requirements. Makes sense.
> 
> For (1) we can just use any old type I think - pfn_t, etc. As we
> discussed in the base series.
> 
> For (2) we need to check that the memory came from the page allocator,
> or otherwise is valid TDX memory somehow. That is at least the only
> check that makes sense to me.
> 
> There was some discussion about refcounts somewhere in this thread. I
> don't think it's arch/x86's worry. Then Yan was saying something last
> night that I didn't quite follow. We said, let's just resume the
> discussion on the list. So she might suggest another check.
Hmm, I previously had a concern about passing "struct page *" as the SEAMCALL
wrapper parameter. For example, when we do sanity checks for valid TDX memory in
tdh_mem_page_aug(), we need to do the sanity check on every page, right?
However, with base_page + npages, it's not easy to get the ith page's pointer
without first ensuring the pages are contained in a single folio. It would also
be superfluous if we first get base_pfn from base_page, and then derive the ith
page from base_pfn + i.

IIUC, this concern should be gone as Dave has agreed to use "pfn" as the
SEAMCALL parameter [1]?
Then should we invoke "KVM_MMU_WARN_ON(!tdx_is_convertible_pfn(pfn));" in KVM
for every pfn of a huge mapping? Or should we keep the sanity check inside the
SEAMCALL wrappers?

BTW, I have another question about the SEAMCALL wrapper implementation, as Kai
also pointed out in [2]: since the SEAMCALL wrappers now serve as APIs available
to callers besides KVM, should the SEAMCALL wrappers return TDX_OPERAND_INVALID
or WARN_ON() (or WARN_ON_ONCE()) on sanity check failure?

By returning TDX_OPERAND_INVALID, the caller can check the return code, adjust
the input or trigger WARN_ON() by itself;
By triggering WARN_ON() directly in the SEAMCALL wrapper, we need to document
this requirement for the SEAMCALL wrappers and have the caller invoke the API
correctly.

So, it looks that "WARN_ON() directly in the SEAMCALL wrapper" is the preferred
approach, right?

[1] https://lore.kernel.org/all/d119c824-4770-41d2-a926-4ab5268ea3a6@intel.com/
[2] https://lore.kernel.org/all/baf6df2cc63d8e897455168c1bf07180fc9c1db8.camel@intel.com