linux-kernel - Re: [PATCHv2] x86/mm: Fix memory encryption features advertisement

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Date: Tue, 16 Jan 2024 21:16:58 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC: "Cui, Dexuan" <decui@...rosoft.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"jpiotrowski@...ux.microsoft.com" <jpiotrowski@...ux.microsoft.com>,
	"hpa@...or.com" <hpa@...or.com>, "mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "thomas.lendacky@....com"
	<thomas.lendacky@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCHv2] x86/mm: Fix memory encryption features advertisement

On Tue, 2024-01-16 at 13:58 +0300, kirill.shutemov@...ux.intel.com wrote:
> On Tue, Jan 16, 2024 at 10:36:10AM +0000, Huang, Kai wrote:
> > On Thu, 2024-01-11 at 14:12 +0300, Kirill A. Shutemov wrote:
> > > When memory encryption is enabled, the kernel prints the encryption
> > > flavor that the system supports.
> > > 
> > > The check assumes that everything is AMD SME/SEV if it doesn't have
> > > the TDX CPU feature set.
> > > 
> > > Hyper-V vTOM sets cc_vendor to CC_VENDOR_INTEL when it runs as L2 guest
> > > on top of TDX, but not X86_FEATURE_TDX_GUEST. Hyper-V only needs memory
> > > encryption enabled for I/O without the rest of CoCo enabling.
> > > 
> > > To avoid confusion, check the cc_vendor directly.
> > > 
> > > Possible alternative is to completely removing the print statement.
> > > For a regular TDX guest, the kernel already prints a message indicating
> > > that it is booting on TDX. Similarly, AMD and Hyper-V can also display
> > > a message during their enumeration process.
> > > 
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > > Cc: Dexuan Cui <decui@...rosoft.com>
> > > Cc: Jeremi Piotrowski <jpiotrowski@...ux.microsoft.com>
> > 
> > Seems this fix is for userspace running in hyperv environment being able to use
> > some easy grep to get which coco vendor it is running on?
> 
> Making decision in userspace by	grepping dmesg is bad idea and nobody
> should do this. It can easily give false result: dmesg is not ABI, format
> can change and ring buffer has finite size, the message can be overridden.
> 
> If we need a way for userspace to discover which CoCo environment it runs
> on, we need proper ABI for that. Maybe sysfs file or something.

Yeah agreed.  :-)