linux-kernel - Re: [PATCH v2 18/25] KVM: TDX: Support per-VM KVM_CAP_MAX

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Message-ID: <08a02ded469a50cc6d0ae3998d9f3e2ba643c7ed.camel@intel.com>
Date: Sun, 5 Jan 2025 22:12:54 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "pbonzini@...hat.com" <pbonzini@...hat.com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "seanjc@...gle.com" <seanjc@...gle.com>, "Edgecombe,
 Rick P" <rick.p.edgecombe@...el.com>
CC: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "tony.lindgren@...ux.intel.com"
	<tony.lindgren@...ux.intel.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Zhao, Yan Y" <yan.y.zhao@...el.com>, "Chatre, Reinette"
	<reinette.chatre@...el.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>
Subject: Re: [PATCH v2 18/25] KVM: TDX: Support per-VM KVM_CAP_MAX_VCPUS
 extension check

On Wed, 2024-10-30 at 12:00 -0700, Rick Edgecombe wrote:
> Note some old modules may also not support this metadata, in which case
> the limit is U16_MAX.

+Dave for a side topic.

I think we should delete this sentence in the new version of this patch since
this sentence is now obsolete which the new patch to read essential metadata for
KVM.

This sentence was needed since originally we had code to do (pseudo):

  if (read_sys_metadata_field(MAX_VCPUS_PER_TD, &td_conf->max_vcpus_per_td))
      td_conf->max_vcpus_per_td = U16_MAX;

Now the above code is removed in the patch which reads essential metadata for
KVM, and reading failure of this metadata will be fatal just like reading
others.

It was removed because when I was trying to avoid special handling in the the
python script when generating the metadata reading code, I found the NO_BRP_MOD
feature was introduced to the module way after the MAX_VCPUS_PER_TD metadata was
added, therefore practically this field will always be present for the modules
that Linux support.

Please let me know if you have different opinion, i.e., we should still do the
old way in the patch which reads essential metadata for KVM?