linux-kernel - Re: [PATCH V4 0/1] KVM: TDX: Decrease TDX VM shutdown time

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Message-ID: <a29d4a7f319f95a45f775270c75ccf136645fad4.camel@intel.com>
Date: Fri, 11 Jul 2025 23:17:20 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Gao, Chao" <chao.gao@...el.com>, "Huang, Kai" <kai.huang@...el.com>,
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, "Li, Xiaoyao"
	<xiaoyao.li@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Hunter, Adrian" <adrian.hunter@...el.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Chatre, Reinette" <reinette.chatre@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "tony.lindgren@...ux.intel.com"
	<tony.lindgren@...ux.intel.com>
Subject: Re: [PATCH V4 0/1] KVM: TDX: Decrease TDX VM shutdown time

On Fri, 2025-07-11 at 16:05 -0700, Sean Christopherson wrote:
> > Zero the reserved area in struct kvm_tdx_capabilities so that fields added
> > in
> > the reserved area won't disturb any userspace that previously had garbage
> > there.
> 
> It's not only about disturbing userspace, it's also about actually being able
> to repurpose the reserved fields in the future without needing *another* flag
> to tell userspace that it's ok to read the previously-reserved fields.  I care
> about this much more than I care about userspace using reserved fields as
> scratch space.

If, before calling KVM_TDX_CAPABILITIES, userspace zeros the new field that it
knows about, but isn't sure if the kernel does, it's the same no?

Did you see that the way KVM_TDX_CAPABILITIES is implemented today is a little
weird? It actually copies the whole struct kvm_tdx_capabilities from userspace
and then sets some fields (not reserved) and then copies it back. So userspace
can zero any fields it wants to know about before calling KVM_TDX_CAPABILITIES.
Then it could know the same things as if the kernel zeroed it.

I was actually wondering if we want to change the kernel to zero reserved, if it
might make more sense to just copy caps->cpuid.nent field from userspace, and
then populate the whole thing starting from a zero'd buffer in the kernel.