linux-kernel - Re: [RFC PATCH v2 6/5] KVM: TDX: Explicitly do WBINVD upon reboot notifier

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Message-ID: <8327b8ebab52a7b38431169193e7281e92e9bf01.camel@intel.com>
Date: Mon, 16 Jun 2025 10:39:24 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "tglx@...utronix.de" <tglx@...utronix.de>, "peterz@...radead.org"
	<peterz@...radead.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>, "Hansen,
 Dave" <dave.hansen@...el.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"bp@...en8.de" <bp@...en8.de>
CC: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "x86@...nel.org" <x86@...nel.org>, "sagis@...gle.com"
	<sagis@...gle.com>, "hpa@...or.com" <hpa@...or.com>, "Chatre, Reinette"
	<reinette.chatre@...el.com>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>, "Williams, Dan J"
	<dan.j.williams@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "thomas.lendacky@....com"
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	"ashish.kalra@....com" <ashish.kalra@....com>, "nik.borisov@...e.com"
	<nik.borisov@...e.com>
Subject: Re: [RFC PATCH v2 6/5] KVM: TDX: Explicitly do WBINVD upon reboot
 notifier

On Fri, 2025-06-13 at 13:36 +0200, Paolo Bonzini wrote:
> On 5/10/25 13:25, Kai Huang wrote:
> > On TDX platforms, during kexec, the kernel needs to make sure there's no
> > dirty cachelines of TDX private memory before booting to the new kernel
> > to avoid silent memory corruption to the new kernel.
> > 
> > During kexec, the kexec-ing CPU firstly invokes native_stop_other_cpus()
> > to stop all remote CPUs before booting to the new kernel.  The remote
> > CPUs will then execute stop_this_cpu() to stop themselves.
> > 
> > The kernel has a percpu boolean to indicate whether the cache of a CPU
> > may be in incoherent state.  In stop_this_cpu(), the kernel does WBINVD
> > if that percpu boolean is true.
> > 
> > TDX turns on that percpu boolean on a CPU when the kernel does SEAMCALL.
> > This makes sure the cahces will be flushed during kexec.
> > 
> > However, the native_stop_other_cpus() and stop_this_cpu() have a "race"
> > which is extremely rare to happen but if did could cause system to hang.
> 
> s/if did//
> 
> > Specifically, the native_stop_other_cpus() firstly sends normal reboot
> > IPI to remote CPUs and wait one second for them to stop.  If that times
> > out, native_stop_other_cpus() then sends NMIs to remote CPUs to stop
> > them.
> > 
> > The aforementioned race happens when NMIs are sent.  Doing WBINVD in
> > stop_this_cpu() makes each CPU take longer time to stop and increases
> > the chance of the race to happen.
> > 
> > Register reboot notifier in KVM to explcitly flush caches upon reboot
> > for TDX.  This brings doing WBINVD at earlier stage and aovids the
> > WBINVD in stop_this_cpu(), eliminating the possibility of increasing the
> > chance of the aforementioned race.
> 
> "This moves the WBINVD to an earlier stage than stop_this_cpus(), 
> avoiding a possibly lengthy operation at a time where it could cause 
> this race."
> 
> Acked-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> Waiting for v3. :)

Thanks Paolo, and will do!