linux-kernel - Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all" the locks

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Message-ID: <77d8a0d9541ce3fc2b2c76b58add50d152b52e39.camel@intel.com>
Date: Mon, 27 Oct 2025 17:46:03 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>
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Subject: Re: [PATCH v3 24/25] KVM: TDX: Guard VM state transitions with "all"
 the locks

On Mon, 2025-10-27 at 17:26 +0800, Yan Zhao wrote:
> > Ugh, I'd rather not?  Refresh me, what's the story with "v1"?  Are we now on
> > v2?
> No... We are now on v1.
> As in [1], I found that TDX module changed SEAMCALL TDH_VP_INIT behavior to
> require exclusive lock on resource TDR when leaf_opcode.version > 0.
> 
> Therefore, we moved KVM_TDX_INIT_VCPU to tdx_vcpu_unlocked_ioctl() in patch
> 22.
> 
> [1] https://lore.kernel.org/all/aLa34QCJCXGLk%2Ffl@yzhao56-desk.sh.intel.com/

Looking at the PDF docs, TDR exclusive locking in version == 1 is called out at
least back to the oldest ABI docs I have (March 2024). Not sure about the
assertion that the behavior changed, but if indeed this was documented, it's a
little bit our bad. We might consider being flexible around calling it a TDX ABI
break?

Sean, can you elaborate why taking mmu_lock is objectionable here, though?

Note, myself (and I think Yan?) determined the locking by examining TDX module
source. For myself, it's possible I misread the locking originally.

Also, I'm not sure about switching gears at this point, but it makes me wonder
about the previously discussed option of trying to just duplicate the TDX locks
on the kernel side. Or perhaps make some kind of debug time lockdep type thing
to document/check the assumptions in the kernel. Something along the lines of
this patch, but to map the TDX locks to KVM locks or something. As we add more
things (DPAMT, etc), it doesn't seem like the TDX locking is getting tamer...