linux-kernel - Re: [PATCH v5 0/2] nosnp sev command line support

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Message-ID: <f751ffe7-9900-4d91-9e7a-e560777725e1@intel.com>
Date: Mon, 30 Sep 2024 16:25:23 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Pavan Kumar Paluri <papaluri@....com>, linux-kernel@...r.kernel.org
Cc: linux-doc@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 Eric Van Tassell <Eric.VanTassell@....com>,
 Tom Lendacky <thomas.lendacky@....com>, Ashish Kalra <ashish.kalra@....com>,
 Michael Roth <michael.roth@....com>, "H . Peter Anvin" <hpa@...or.com>,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v5 0/2] nosnp sev command line support

On 9/30/24 16:11, Pavan Kumar Paluri wrote:
> Provide "nosnp" boot option via "sev=nosnp" kernel command line to
> prevent SEV-SNP[1] capable host kernel from enabling SEV-SNP and
> initializing Reverse Map Table (RMP) [1].
> 
> On providing sev=nosnp via kernel command line:
> cat /sys/module/kvm_amd/parameters/sev_snp should be "N".

I don't see any mention in the changelog, cover letter or Documentation/
about why someone would want to do this.

I assume it's because of performance (walking the RMP table is non-zero
cost).

The BIOS allocates the RMP table, right?  So this option presumably gets
the performance back, but not the memory.  That's probably also worth
mentioning ... somewhere.