linux-kernel - Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on demand

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Message-ID: <1a8099e2-da28-6b2a-7b5a-1d6346b7f95d@intel.com>
Date:   Wed, 5 Jul 2023 07:34:06 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Isaku Yamahata <isaku.yamahata@...il.com>,
        Kai Huang <kai.huang@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Ashok Raj <ashok.raj@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        "david@...hat.com" <david@...hat.com>,
        "bagasdotme@...il.com" <bagasdotme@...il.com>,
        "ak@...ux.intel.com" <ak@...ux.intel.com>,
        Rafael J Wysocki <rafael.j.wysocki@...el.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
        "nik.borisov@...e.com" <nik.borisov@...e.com>,
        "hpa@...or.com" <hpa@...or.com>, Sagi Shahar <sagis@...gle.com>,
        "imammedo@...hat.com" <imammedo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>, Chao Gao <chao.gao@...el.com>,
        Len Brown <len.brown@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Ying Huang <ying.huang@...el.com>,
        Dan J Williams <dan.j.williams@...el.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on
 demand

On 7/4/23 09:58, Peter Zijlstra wrote:
> If we have concerns about allocating the PAMT array, can't we use CMA
> for this? Allocate the whole thing at boot as CMA such that when not
> used for TDX it can be used for regular things like userspace and
> filecache pages?

I never thought of CMA as being super reliable.  Maybe it's improved
over the years.

KVM also has a rather nasty habit of pinning pages, like for device
passthrough.  I suspect that means that we'll have one of two scenarios:

 1. CMA works great, but the TDX/CMA area is unusable for KVM because
    it's pinning all its pages and they just get moved out of the CMA
    area immediately.  The CMA area is effectively wasted.
 2. CMA sucks, and users get sporadic TDX failures when they wait a long
    time to run a TDX guest after boot.  Users just work around the CMA
    support by starting up TDX guests at boot or demanding a module
    parameter be set.  Hacking in CMA support was a waste.

Am I just too much of a pessimist?