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

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Message-ID: <aBEJVzesMum9-Rem@google.com>
Date: Tue, 29 Apr 2025 10:16:07 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
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, Peter Zijlstra wrote:
> 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.

FWIW, you can do the same with KVM selftests (and presumably others?), although
the syntax is kinda weird (no idea why lib.mk uses OUTPUT instead of O).

E.g. to build KVM selftests in $HOME/build/selftests/x86

  make O=$HOME/build/selftests/x86 headers_install
  make OUTPUT=$HOME/build/selftests/x86