linux-kernel - Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem config combination

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Message-ID: <CAMj1kXFk=tBjPaCJ_f0aiw2K3i8XZehud91y1s-WYc6eRiMVtA@mail.gmail.com>
Date: Fri, 28 Mar 2025 17:18:44 +0100
From: Ard Biesheuvel <ardb@...nel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>, Steven Rostedt <rostedt@...dmis.org>, 
	Tom Lendacky <thomas.lendacky@....com>, "Aithal, Srikanth" <sraithal@....com>, 
	Jason Baron <jbaron@...mai.com>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, Linux-Next Mailing List <linux-next@...r.kernel.org>, 
	open list <linux-kernel@...r.kernel.org>, "Roth, Michael" <Michael.Roth@....com>
Subject: Re: linux-next regression: SNP Guest boot hangs with certain cpu/mem
 config combination

On Fri, 28 Mar 2025 at 11:54, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> On Fri, Mar 28, 2025 at 10:33:31AM +0100, Ard Biesheuvel wrote:
> > Can you quantify the speedup?
>
> Test is below. I run it 10 times on a VM without unaccepted memory. With
> and without has_unaccepted_memory() check in cond_accept_memory().
>
> The difference is not huge, but it is there:
>
> Without static branch:  Mean: 35559993 us, StdDev: 167264
> With static branch:     Mean: 35286227 us, StdDev: 207595
> Diff:                   -273766 us / -0.77%
>

Fair enough - I think this is pretty close to negligible, but I know
that other people may feel differently. At least we have documented
the justification for using a static key here.