linux-kernel - Re: [PATCH v3 00/21] TDX host kernel support

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Date:   Fri, 29 Apr 2022 07:39:52 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Dan Williams <dan.j.williams@...el.com>,
        Kai Huang <kai.huang@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Brown, Len" <len.brown@...el.com>,
        "Luck, Tony" <tony.luck@...el.com>,
        Rafael J Wysocki <rafael.j.wysocki@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [PATCH v3 00/21] TDX host kernel support

On 4/28/22 19:58, Dan Williams wrote:
> That only seems possible if the kernel is given a TDX capable physical
> address map at the beginning of time.

TDX actually brings along its own memory map.  The "EAS"[1]. has a lot
of info on it, if you know where to find it.  Here's the relevant chunk:

CMR - Convertible Memory Range -
	A range of physical memory configured by BIOS and verified by
	MCHECK. MCHECK verificatio n is intended to help ensure that a
	CMR may be used to hold TDX memory pages encrypted with a
	private HKID.

So, the BIOS has the platform knowledge to enumerate this range.  It
stashes the information off somewhere that the TDX module can find it.
Then, during OS boot, the OS makes a SEAMCALL (TDH.SYS.CONFIG) to the
TDX module and gets the list of CMRs.

The OS then has to reconcile this CMR "memory map" against the regular
old BIOS-provided memory map, tossing out any memory regions which are
RAM, but not covered by a CMR, or disabling TDX entirely.

Fun, eh?

I'm still grappling with how this series handles the policy of what
memory to throw away when.

1.
https://www.intel.com/content/dam/develop/external/us/en/documents/intel-tdx-module-1eas.pdf