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

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Message-ID: <aXzPIO2qZwuwaeLi@google.com>
Date: Fri, 30 Jan 2026 07:32:48 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, Fan Du <fan.du@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Kai Huang <kai.huang@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, Dave Hansen <dave.hansen@...el.com>, 
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "vbabka@...e.cz" <vbabka@...e.cz>, 
	"tabba@...gle.com" <tabba@...gle.com>, "david@...nel.org" <david@...nel.org>, "kas@...nel.org" <kas@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Ira Weiny <ira.weiny@...el.com>, 
	"francescolavra.fl@...il.com" <francescolavra.fl@...il.com>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "nik.borisov@...e.com" <nik.borisov@...e.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Chao P Peng <chao.p.peng@...el.com>, "michael.roth@....com" <michael.roth@....com>, 
	Vishal Annapurve <vannapurve@...gle.com>, "sagis@...gle.com" <sagis@...gle.com>, Chao Gao <chao.gao@...el.com>, 
	Jun Miao <jun.miao@...el.com>, "jgross@...e.com" <jgross@...e.com>, 
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 00/24] KVM: TDX huge page support for private memory

On Mon, Jan 19, 2026, Yan Zhao wrote:
> 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:
> 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?

I don't have a strong preference.  But if it goes in KVM, definitely guard it with
KVM_MMU_WARN_ON().

> 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?

Why not both?  But maybe TDX_SW_ERROR instead of TDX_OPERAND_INVALID?

If an API has a defined contract and/or set of expectations, and those expectations
aren't met by the caller, then a WARN is justified.  But the failure still needs
to be communicated to the caller.

> 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.

Document what exactly?  Most of this should be common sense.  E.g. we don't generally
document that pointers must be non-NULL, because that goes without saying 99.9%
of the time.

IMO, that holds true here as well.  E.g. trying to map memory into a TDX guest
that isn't convertible is obviously a bug, I don't see any value in formally
documenting that requirement.

> 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