linux-kernel - Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more TDCALL/SEAMCALL leafs

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Message-ID: <20230713084640.GB3138667@hirez.programming.kicks-ass.net>
Date:   Thu, 13 Jul 2023 10:46:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
 TDCALL/SEAMCALL leafs

On Thu, Jul 13, 2023 at 07:48:20AM +0000, Huang, Kai wrote:

> I found below comment in KVM code:
> 
> > +	 * See arch/x86/kvm/vmx/vmenter.S:
> > +	 *
> > +	 * In theory, a L1 cache miss when restoring register from stack
> > +	 * could lead to speculative execution with guest's values.
> 
> And KVM explicitly does XOR for the registers that gets "pop"ed almost
> instantly, so I followed.
> 
> But to be honest I don't quite understand this.  :-)

Urgh, I suppose that actually makes sense. Since pop is a load it might
continue speculation with the previous value. Whereas the xor-clear
idiom is impossible to speculate through.

Oh well...