linux-kernel - Re: [PATCH 4/5] x86/virt/tdx: Support global metadata read for all element sizes

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Date: Fri, 3 May 2024 13:14:22 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
CC: "Hansen, Dave" <dave.hansen@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org"
	<x86@...nel.org>, "peterz@...radead.org" <peterz@...radead.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>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "jgross@...e.com" <jgross@...e.com>, "Yamahata, Isaku"
	<isaku.yamahata@...el.com>
Subject: Re: [PATCH 4/5] x86/virt/tdx: Support global metadata read for all
 element sizes



On 3/05/2024 12:19 pm, Edgecombe, Rick P wrote:
> On Sat, 2024-03-02 at 00:20 +1300, Kai Huang wrote:
>> For now the kernel only reads TDMR related global metadata fields for
>> module initialization.  All these fields are 16-bits, and the kernel
>> only supports reading 16-bits fields.
>>
>> KVM will need to read a bunch of non-TDMR related metadata to create and
>> run TDX guests.  It's essential to provide a generic metadata read
>> infrastructure which supports reading all 8/16/32/64 bits element sizes.
>>
>> Extend the metadata read to support reading all these element sizes.
> 
> It makes it sound like KVM needs 8 bit fields. I think it only needs 16 and 64.
> (need to verify fully) But the code to support those can be smaller if it's
> generic to all sizes.
> 
> It might be worth mentioning which fields and why to make it generic. To make
> sure it doesn't come off as a premature implementation.

How about:

For now the kernel only reads TDMR related global metadata fields for
module initialization.  All these fields are 16-bits, and the kernel
only supports reading 16-bits fields.

KVM will need to read a bunch of non-TDMR related metadata to create and
run TDX guests, and KVM will at least need to additionally be able to 
read 64-bit metadata fields.

For example, the KVM will need to read the 64-bit ATTRIBUTES_FIXED{0|1} 
fields to determine whether a particular attribute is legal when 
creating a TDX guest.  Please see 'global_metadata.json in [1] for more 
information.

To resolve this once for all, extend the existing metadata reading code 
to provide a generic metadata read infrastructure which supports reading 
all 8/16/32/64 bits element sizes.

[1] https://cdrdv2.intel.com/v1/dl/getContent/795381