linux-kernel - Re: [RFC PATCH v2 23/23] KVM: TDX: Turn on PG_LEVEL

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Message-ID: <aRbpnPdgh1h4SDAu@yzhao56-desk.sh.intel.com>
Date: Fri, 14 Nov 2025 16:34:36 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "Du, Fan" <fan.du@...el.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "vbabka@...e.cz"
	<vbabka@...e.cz>, "tabba@...gle.com" <tabba@...gle.com>, "kas@...nel.org"
	<kas@...nel.org>, "michael.roth@....com" <michael.roth@....com>, "Weiny, Ira"
	<ira.weiny@...el.com>, "linux-kernel@...r.kernel.org"
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	"Peng, Chao P" <chao.p.peng@...el.com>, "Annapurve, Vishal"
	<vannapurve@...gle.com>, "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"Miao, Jun" <jun.miao@...el.com>, "x86@...nel.org" <x86@...nel.org>,
	"pgonda@...gle.com" <pgonda@...gle.com>
Subject: Re: [RFC PATCH v2 23/23] KVM: TDX: Turn on PG_LEVEL_2M after TD is
 RUNNABLE

On Tue, Nov 11, 2025 at 07:25:30PM +0800, Huang, Kai wrote:
> On Thu, 2025-08-07 at 17:46 +0800, Yan Zhao wrote:
> > +	/* Large page is not supported before TD runnable,*/
> > +	if (KVM_BUG_ON(kvm_tdx->state != TD_STATE_RUNNABLE && level != PG_LEVEL_4K, kvm))
> >  		return -EINVAL;
> 
> Not a particular comment to this patch, but could you elaborate a little bit
> why PROMOTE isn't supported in this series?  This doesn't seem to be
> mentioned anywhere in this series (not in the coverletter either).
I mentioned it briefly in the coverletter:

6. Page merging (page promotion)
   Promotion is disallowed (in patch 7), because
   - The current TDX module requires all 4KB leafs to be either all PENDING
     or all ACCEPTED before a successful promotion to 2MB. This requirement
     prevents successful page merging after partially converting a 2MB
     range from private to shared and then back to private, which is the
     primary scenario necessitating page promotion.
   - tdh_mem_page_promote() depends on tdh_mem_range_block() in the current
     TDX module. Consequently, handling BUSY errors is complex, as page
     merging typically occurs in the fault path under a shared mmu_lock.


v1 explains it in more details (See section "Page merging (page promotion)" in
[*]).

[*] https://lore.kernel.org/all/20250424030033.32635-1-yan.y.zhao@intel.com/

> E.g., theoretically, I think we can have a way to PROMOTE mappings for
> initial memory pages (via TDH.MEM.PAGE.ADD), e.g., right before the TD is
> becoming runnable?
Right. Kirill also asked it in in v1 [1].

Though we have no need to worry about the nr_premapped calculation after Sean's
cleanup series, I think there's no need to complicate the design for the initial
support, due to the limited the amount of initial memory pages.

In my environment, for a TD with 8GB memory, there are 1086 count of 2MB mapping
at runtime, but the initial memory is merely 1049 4KB pages in total.
So, the gain is less than 2/1000.

Will call it out in the next version.

[1] https://lore.kernel.org/all/aAn3SSocw0XvaRye@yzhao56-desk.sh.intel.com/