linux-kernel - Re: [PATCH 3/6] x86/kvm/emulate: Avoid RET for fastops

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Message-ID: <20250429144631.GI4198@noisy.programming.kicks-ass.net>
Date: Tue, 29 Apr 2025 16:46:31 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org, kys@...rosoft.com,
	haiyangz@...rosoft.com, wei.liu@...nel.org, decui@...rosoft.com,
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
	dave.hansen@...ux.intel.com, hpa@...or.com,
	pawan.kumar.gupta@...ux.intel.com, pbonzini@...hat.com,
	ardb@...nel.org, kees@...nel.org, Arnd Bergmann <arnd@...db.de>,
	gregkh@...uxfoundation.org, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-efi@...r.kernel.org, samitolvanen@...gle.com,
	ojeda@...nel.org, shuah@...nel.org
Subject: Re: [PATCH 3/6] x86/kvm/emulate: Avoid RET for fastops

On Tue, Apr 29, 2025 at 07:05:35AM -0700, Sean Christopherson wrote:
> On Tue, Apr 29, 2025, Peter Zijlstra wrote:
> > On Mon, Apr 28, 2025 at 10:13:31AM -0700, Sean Christopherson wrote:
> > > On Sat, Apr 26, 2025, Peter Zijlstra wrote:
> > > > On Wed, Apr 16, 2025 at 10:38:59AM +0200, Peter Zijlstra wrote:
> > > > 
> > > > > Yeah, I finally got there. I'll go cook up something else.
> > > > 
> > > > Sean, Paolo, can I once again ask how best to test this fastop crud?
> > > 
> > > Apply the below, build KVM selftests, 
> > 
> > Patch applied, my own hackery applied, host kernel built and booted,
> > foce_emulation_prefix set, but now I'm stuck at this seemingly simple
> > step..
> > 
> > $ cd tools/testing/selftests/kvm/
> > $ make
> > ... metric ton of fail ...
> > 
> > Clearly I'm doing something wrong :/
> 
> Did you install headers in the top level directory?  I.e. make headers_install.

No, of course not :-) I don't use the top directory to build anything,
ever.

All my builds are into build directories, using make O=foo. This allows
me to do parallel builds for multiple architectures etc. Also, much
easier to wipe a complete build directory than it is to clean out the
top level dir.

> The selftests build system was change a while back to require users to manually
> install headers (I forget why, but it is indeed annoying).

Bah, I remember NAK-ing that. Clearly the selftest people don't want
selftests to be usable :-(

Anyway, mingo build me a copy of the fastop selftest, and aside from a
few stupid mistakes, I seem to now pass it \o/

I'll attempt a Changelog and update the hyper-v patches and then post
the lot.