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

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Message-ID: <8e2f8e880269724d5f39933f8566140e9c4555c5.camel@intel.com>
Date: Tue, 18 Nov 2025 00:56:50 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Du, Fan" <fan.du@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen, Dave"
	<dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
	"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
	<tabba@...gle.com>, "vbabka@...e.cz" <vbabka@...e.cz>, "michael.roth@....com"
	<michael.roth@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "seanjc@...gle.com" <seanjc@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "binbin.wu@...ux.intel.com"
	<binbin.wu@...ux.intel.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "kas@...nel.org" <kas@...nel.org>, "Weiny, Ira"
	<ira.weiny@...el.com>, "Peng, Chao P" <chao.p.peng@...el.com>, "Yamahata,
 Isaku" <isaku.yamahata@...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 Fri, 2025-11-14 at 16:34 +0800, Yan Zhao wrote:
> 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/

Agreed.  Thanks.