linux-kernel - Re: [PATCH v3 3/8] x86, lib: Add WBNOINVD helper functions

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Message-ID: <20250711085219.GA3108775@noisy.programming.kicks-ass.net>
Date: Fri, 11 Jul 2025 10:52:19 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Borislav Petkov <bp@...en8.de>
Cc: Sean Christopherson <seanjc@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Kai Huang <kai.huang@...el.com>, Ingo Molnar <mingo@...nel.org>,
	Zheyun Shen <szy0127@...u.edu.cn>,
	Mingwei Zhang <mizhang@...gle.com>,
	Francesco Lavra <francescolavra.fl@...il.com>
Subject: Re: [PATCH v3 3/8] x86, lib: Add WBNOINVD helper functions

On Thu, Jul 10, 2025 at 05:47:05PM +0200, Peter Zijlstra wrote:

> So kvm-amd is the SEV stuff, AGPGART is the ancient crap nobody cares
> about, CCP is more SEV stuff, DRM actually does CLFLUSH loops, but has a
> WBINVD fallback. i915 is rude and actually does WBINVD. Could they
> pretty please also do CLFLUSH loops?

So having looked at i915 a little more:

drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c:      * Currently we just do a heavy handed wbinvd_on_all_cpus() here since
drivers/gpu/drm/i915/gem/i915_gem_dmabuf.c:             wbinvd_on_all_cpus();

This one can be runtime; but is only applied on certain parts that might
be less than coherent -- hopefully not new parts, but who knows.

Parts not taking this path end up calling drm_clflush_sg(), which DTRT.


And these all look suspend/resume related, so we can live with them:

drivers/gpu/drm/i915/gem/i915_gem_pm.c:#define wbinvd_on_all_cpus() \
drivers/gpu/drm/i915/gem/i915_gem_pm.c:         wbinvd_on_all_cpus();
drivers/gpu/drm/i915/gem/i915_gem_pm.c: wbinvd_on_all_cpus();
drivers/gpu/drm/i915/gt/intel_ggtt.c:           wbinvd_on_all_cpus();