Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot

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Subject: Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Date: Tue, 20 Jun 2023 12:56:12 +0200
Cc: "kvm@xxxxxxxxxxxxxxx" <kvm@xxxxxxxxxxxxxxx>, "Hansen, Dave" <dave.hansen@xxxxxxxxx>, "david@xxxxxxxxxx" <david@xxxxxxxxxx>, "bagasdotme@xxxxxxxxx" <bagasdotme@xxxxxxxxx>, "ak@xxxxxxxxxxxxxxx" <ak@xxxxxxxxxxxxxxx>, "Wysocki, Rafael J" <rafael.j.wysocki@xxxxxxxxx>, "kirill.shutemov@xxxxxxxxxxxxxxx" <kirill.shutemov@xxxxxxxxxxxxxxx>, "Chatre, Reinette" <reinette.chatre@xxxxxxxxx>, "Christopherson,, Sean" <seanjc@xxxxxxxxxx>, "pbonzini@xxxxxxxxxx" <pbonzini@xxxxxxxxxx>, "tglx@xxxxxxxxxxxxx" <tglx@xxxxxxxxxxxxx>, "Yamahata, Isaku" <isaku.yamahata@xxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, "linux-mm@xxxxxxxxx" <linux-mm@xxxxxxxxx>, "Luck, Tony" <tony.luck@xxxxxxxxx>, "Shahar, Sagi" <sagis@xxxxxxxxxx>, "imammedo@xxxxxxxxxx" <imammedo@xxxxxxxxxx>, "Gao, Chao" <chao.gao@xxxxxxxxx>, "Brown, Len" <len.brown@xxxxxxxxx>, "sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx" <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>, "Huang, Ying" <ying.huang@xxxxxxxxx>, "Williams, Dan J" <dan.j.williams@xxxxxxxxx>
In-reply-to: <93b49134cdf5c7f0658ba0aeb2b1a49778b2aece.camel@intel.com>
References: <cover.1685887183.git.kai.huang@intel.com> <5aa7506d4fedbf625e3fe8ceeb88af3be1ce97ea.1685887183.git.kai.huang@intel.com> <20230609132301.uvvp27yr5kpenl6f@box.shutemov.name> <58f34b4b81b6d6b37d3386dec0f073e6eb7a97ff.camel@intel.com> <20230620081131.GV4253@hirez.programming.kicks-ass.net> <93b49134cdf5c7f0658ba0aeb2b1a49778b2aece.camel@intel.com>

On Tue, Jun 20, 2023 at 10:42:32AM +0000, Huang, Kai wrote:
> On Tue, 2023-06-20 at 10:11 +0200, Peter Zijlstra wrote:
> > On Mon, Jun 12, 2023 at 03:06:48AM +0000, Huang, Kai wrote:
> > 
> > > +       __mb();
> > 
> > __mb() is not a valid interface to use.
> 
> Thanks for feedback!
> 
> May I ask why, for education purpose? :)

it's the raw MFENCE wrapper, not one of the *many* documented barriers.

Also, typicaly you do *not* want MFENCE, MFENCE bad.

References:
- [PATCH v11 00/20] TDX host kernel support
  - From: Kai Huang
- [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
  - From: Kai Huang
- Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
  - From: kirill . shutemov
- Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
  - From: Huang, Kai
- Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
  - From: Peter Zijlstra
- Re: [PATCH v11 18/20] x86: Handle TDX erratum to reset TDX private memory during kexec() and reboot
  - From: Huang, Kai

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