linux-kernel - Re: [PATCH v3 0/3] VMSCAPE optimization for BHI variant

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Message-ID: <20251103230353.ifsayclvtw7bzyga@desk>
Date: Mon, 3 Nov 2025 15:03:53 -0800
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	David Kaplan <david.kaplan@....com>,
	Sean Christopherson <seanjc@...gle.com>,
	Paolo Bonzini <pbonzini@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Asit Mallick <asit.k.mallick@...el.com>,
	Tao Zhang <tao1.zhang@...el.com>
Subject: Re: [PATCH v3 0/3] VMSCAPE optimization for BHI variant

On Mon, Nov 03, 2025 at 12:07:30PM -0800, Dave Hansen wrote:
> On 10/27/25 16:43, Pawan Gupta wrote:
> > | iPerf user-net | IBPB    | BHB Clear |
> > |----------------|---------|-----------|
> > | UDP 1-vCPU_p1  | -12.5%  |   1.3%    |
> ...
> 
> Could you clarify what "1.3%" means? Is that relative to the baseline,
> or relative to the IBPB number?

This is relative to the baseline, sorry I didn't mention that explicitly.

> If it's relative to the baseline, then this data either looks wrong or
> noisy since there are a lot of places where adding the BHB Clear loop
> makes things faster.

I will double check, but I am fairly positive that this wasn't noisy.
Surprisingly, there were a few other cases where the BHB-clearing was
performing better than the baseline.