linux-kernel - Re: [PATCH v3 09/14] virt: sev-guest: Choose the VMPCK key based on executing VMPL

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Message-ID: <661e9daa4b37c_4d56129418@dwillia2-mobl3.amr.corp.intel.com.notmuch>
Date: Tue, 16 Apr 2024 08:47:54 -0700
From: Dan Williams <dan.j.williams@...el.com>
To: Tom Lendacky <thomas.lendacky@....com>, Dan Williams
	<dan.j.williams@...el.com>, <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<linux-coco@...ts.linux.dev>, <svsm-devel@...onut-svsm.dev>
CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>, "Peter
 Zijlstra" <peterz@...radead.org>, Michael Roth <michael.roth@....com>, Ashish
 Kalra <ashish.kalra@....com>
Subject: Re: [PATCH v3 09/14] virt: sev-guest: Choose the VMPCK key based on
 executing VMPL

Tom Lendacky wrote:
> On 4/15/24 23:54, Dan Williams wrote:
> > Hey, Tom, came looking to review the tsm_report changes and noticed
> > this...
> > 
> > Tom Lendacky wrote:
> >> Currently, the sev-guest driver uses the vmpck-0 key by default. When an
> >> SVSM is present the kernel is running at a VMPL other than 0 and the
> >> vmpck-0 key is no longer available. So choose the vmpck key based on the
> >> active VMPL level.
> > 
> > The module parameter is not mentioned in the changelog. Is it not
> > sufficient to always use snp_get_vmpl(), and if not should there be some
> > documentation about when to specify vmpck_id?
> 
> It is possible to encounter an issue that causes the vmpck key to be 
> cleared. In that situation, the guest is allowed to use a vmpck key 
> associated with a lower VMPL. For that reason, the module parameter was 
> added to the driver when it was initially created.

Oh, sorry, misread that the module parameter was not new.

> I can update the changelog to mention this.

I guess it is too late now, but a proper sysfs attribute rather than a
module parameter would let you do sanity checking on the writes and
allow a natural place to document behavior in
Documentation/ABI/testing/sysfs-devices-sev-guest.