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

[Peter Zijlstra <peterz@infradead.org>]

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();