linux-kernel - Re: [PATCH RFC 1/2] KVM: TDX: Disable general support for MWAIT in guest

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Message-ID: <968d2750-cbd6-47cb-b2fc-d0894662dafc@intel.com>
Date: Tue, 19 Aug 2025 10:38:12 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "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>, "Chatre, Reinette"
	<reinette.chatre@...el.com>, "Li, Xiaoyao" <xiaoyao.li@...el.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"tony.lindgren@...ux.intel.com" <tony.lindgren@...ux.intel.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Zhao, Yan Y"
	<yan.y.zhao@...el.com>, "Weiny, Ira" <ira.weiny@...el.com>, "Brown, Len"
	<len.brown@...el.com>
Subject: Re: [PATCH RFC 1/2] KVM: TDX: Disable general support for MWAIT in
 guest

On 18/08/2025 21:49, Edgecombe, Rick P wrote:
> Attn: Binbin, Xiaoyao
> 
> On Mon, 2025-08-18 at 07:05 -0700, Sean Christopherson wrote:
>> NAK.
>>
>> Fix the guest, or wherever else in the pile there are issues.  KVM is NOT carrying
>> hack-a-fixes to workaround buggy software/firmware.  Been there, done that.
> 
> Yes, I would have thought we should have at least had a TDX module change option
> for this.

That would not help with existing TDX Modules, and would possibly require
a guest opt-in, which would not help with existing guests.  Hence, to start
with disabling the feature first, and look for another solution second.

In the MWAIT case, Sean has rejected supporting MSR_PKG_CST_CONFIG_CONTROL
even for VMX, because it is an optional MSR, so altering intel_idle is
being proposed.

> 
> But side topic. We have an existing arch TODO around creating some guidelines
> around how CPUID bit configuration should evolve.
> 
> A new directly configurable CPUID bit that affects host state is an obvious no-
> no. But how about a directly configurable bit that can't hurt the host, but
> requires host changes to virtualize in an x86 arch compliant way? (not quite
> like this MWAIT case)

It is still "new stuff that breaks old stuff" which is generally
"just don't do that".