linux-kernel - Re: [PATCH v6 7/7] KVM: TDX: Explicitly do WBINVD when no more TDX SEAMCALLs

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Message-ID: <aJ4kWcuyNIpCnaXE@google.com>
Date: Thu, 14 Aug 2025 11:00:57 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: Kai Huang <kai.huang@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"ashish.kalra@....com" <ashish.kalra@....com>, Dave Hansen <dave.hansen@...el.com>, 
	"thomas.lendacky@....com" <thomas.lendacky@....com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Reinette Chatre <reinette.chatre@...el.com>, 
	"dwmw@...zon.co.uk" <dwmw@...zon.co.uk>, "pbonzini@...hat.com" <pbonzini@...hat.com>, 
	"mingo@...hat.com" <mingo@...hat.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	"kas@...nel.org" <kas@...nel.org>, "tglx@...utronix.de" <tglx@...utronix.de>, 
	"nik.borisov@...e.com" <nik.borisov@...e.com>, "hpa@...or.com" <hpa@...or.com>, 
	"peterz@...radead.org" <peterz@...radead.org>, "sagis@...gle.com" <sagis@...gle.com>, 
	Farrah Chen <farrah.chen@...el.com>, "bp@...en8.de" <bp@...en8.de>, Chao Gao <chao.gao@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, Dan J Williams <dan.j.williams@...el.com>, 
	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v6 7/7] KVM: TDX: Explicitly do WBINVD when no more TDX SEAMCALLs

On Thu, Aug 14, 2025, Rick P Edgecombe wrote:
> On Thu, 2025-08-14 at 06:54 -0700, Sean Christopherson wrote:
> > > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > > index 66744f5768c8..1bc6f52e0cd7 100644
> > > --- a/arch/x86/kvm/vmx/tdx.c
> > > +++ b/arch/x86/kvm/vmx/tdx.c
> > > @@ -442,6 +442,18 @@ void tdx_disable_virtualization_cpu(void)
> > >   		tdx_flush_vp(&arg);
> > >   	}
> > >   	local_irq_restore(flags);
> > > +
> > > +	/*
> > > +	 * No more TDX activity on this CPU from here.  Flush cache to
> > > +	 * avoid having to do WBINVD in stop_this_cpu() during kexec.
> > > +	 *
> > > +	 * Kexec calls native_stop_other_cpus() to stop remote CPUs
> > > +	 * before booting to new kernel, but that code has a "race"
> > > +	 * when the normal REBOOT IPI times out and NMIs are sent to
> > > +	 * remote CPUs to stop them.  Doing WBINVD in stop_this_cpu()
> > > +	 * could potentially increase the possibility of the "race".

Why is that race problematic?  The changelog just says

 : However, the native_stop_other_cpus() and stop_this_cpu() have a "race"
 : which is extremely rare to happen but could cause the system to hang.
 : 
 : Specifically, the native_stop_other_cpus() firstly sends normal reboot
 : IPI to remote CPUs and waits one second for them to stop.  If that times
 : out, native_stop_other_cpus() then sends NMIs to remote CPUs to stop
 : them.

without explaining how that can cause a system hang.

> > > +	 */
> > > +	tdx_cpu_flush_cache();
> > 
> > IIUC, this can be:
> > 
> > 	if (IS_ENABLED(CONFIG_KEXEC))
> > 		tdx_cpu_flush_cache();
> > 
> 
> No strong objection, just 2 cents. I bet !CONFIG_KEXEC && CONFIG_INTEL_TDX_HOST
> kernels will be the minority. Seems like an opportunity to simplify the code.

Reducing the number of lines of code is not always a simplification.  IMO, not
checking CONFIG_KEXEC adds "complexity" because anyone that reads the comment
(and/or the massive changelog) will be left wondering why there's a bunch of
documentation that talks about kexec, but no hint of kexec considerations in the
code.