linux-kernel - Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

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Message-ID: <97cdcbd6ba0305fd3875813e46b6f625dde0d0d3.camel@intel.com>
Date: Wed, 25 Jun 2025 14:48:00 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "Huang, Kai" <kai.huang@...el.com>, "Du, Fan"
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	"david@...hat.com" <david@...hat.com>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "Li,
 Zhiquan1" <zhiquan1.li@...el.com>, "Shutemov, Kirill"
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Subject: Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is
 RUNNABLE

On Wed, 2025-06-25 at 17:36 +0800, Yan Zhao wrote:
> On Wed, Jun 25, 2025 at 05:28:22PM +0800, Yan Zhao wrote:
> > Write down my understanding to check if it's correct:
> > 
> > - when a TD is NOT configured to support KVM_LPAGE_GUEST_INHIBIT TDVMCALL, KVM
> >    always maps at 4KB
> > 
> > - When a TD is configured to support KVM_LPAGE_GUEST_INHIBIT TDVMCALL,
> Sorry, the two conditions are stale ones. No need any more.
> So it's always
>  
>  (a)
>  1. guest accepts at 4KB
>  2. TDX sets KVM_LPAGE_GUEST_INHIBIT and try splitting.(with write mmu_lock)
>  3. KVM maps at 4KB (with read mmu_lock)
>  4. guest's 4KB accept succeeds.

Yea.

>  
>  (b)
>  1. guest accepts at 2MB.
>  2. KVM maps at 4KB due to a certain reason.

I don't follow this part. You mean because it spans a memslot or other?
Basically that KVM won't guarantee the page size at exactly the accept size? I
think this is ok and good. The ABI can be that KVM will guarantee the S-EPT
mapping size <= the accept size.

>  3. guest's accept 2MB fails with TDACCEPT_SIZE_MISMATCH.
>  4. guest accepts at 4KB
>  5. guest's 4KB accept succeeds.
>  
In this option accept behavior doesn't need to change, but the
TDACCEPT_SIZE_MISMATCH in step 3 still is a little weird. TDX module could
accept at 4k mapping size. But this is an issue for the guest to deal with, not
KVM.