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

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Message-ID: <aYG9ZyvpS6AZiRAl@yzhao56-desk.sh.intel.com>
Date: Tue, 3 Feb 2026 17:18:31 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.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 Fri, Jan 30, 2026 at 07:32:48AM -0800, Sean Christopherson wrote:
> 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().
Thank you for your insights, Sean!

> > 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?
Hmm, I previously returned TDX_OPERAND_INVALID for non-aligned base PFN.
TDX_SW_ERROR is also ok if we want to indicate that passing an invalid PFN is a
software error.
(I had tdh_mem_page_demote() return TDX_SW_ERROR when an incompatible TDX module
is used, i.e., when !tdx_supports_demote_nointerrupt()).

> 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.
Ok.

The reason for 'not both' is that there's already TDX_BUG_ON_2() in KVM after
the SEAMCALL wrapper returns a non-BUSY error. I'm not sure if having double
WARN_ON_ONCE() calls is good, so I intended to let the caller decide whether to
warn.

> > 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.
Document the SEAMCALL wrapper's expectations. e.g., for demote, a PFN must be
2MB-aligned, or the caller must not invoke tdh_mem_page_demote() if a TDX module
does not support feature ENHANCED_DEMOTE_INTERRUPTIBILITY...

> 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.
Do we need a comment for documentation above the tdh_mem_page_demote() API?

> > 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
>