RE: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

From: "Liu, Jing2" <jing2.liu@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>
Cc: "x86@kernel.org" <x86@kernel.org>,
	"Bae, Chang Seok" <chang.seok.bae@intel.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"Arjan van de Ven" <arjan@linux.intel.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
	"Nakajima, Jun" <jun.nakajima@intel.com>,
	Jing Liu <jing2.liu@linux.intel.com>,
	"seanjc@google.com" <seanjc@google.com>,
	"Cooper, Andrew" <andrew.cooper3@citrix.com>
Subject: RE: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
Date: Fri, 15 Oct 2021 13:01:57 +0000	[thread overview]
Message-ID: <BYAPR11MB3256EAFF1478F4C5D2372047A9B99@BYAPR11MB3256.namprd11.prod.outlook.com> (raw)
In-Reply-To: <877dee5zpf.ffs@tglx>

Hi Thomas,
On 10/15/2021 6:50 PM, Thomas Gleixner wrote:
> Jing,
> 
> On Fri, Oct 15 2021 at 09:00, Jing2 Liu wrote:
> > On 10/14/2021 11:01 PM, Paolo Bonzini wrote:
> > For the guest dynamic state support, based on the latest discussion,
> > four copies of XFD need be cared and switched, I'd like to list as
> > follows.
> 
> There will not be 4 copies. Read my last mail and think about the
> consequences.
> 
Actually I saw there are fpu_init_fpstate_user(vcpu->arch.user_fpu)
and fpu_init_fpstate_user(vcpu->arch.guest_fpu) in the full series,
so I understood that we'd keep it this way. (Your last mail corrects me)

But yes, these xstate copies really make things complex and bad,
and I'm glad to do for a good clean way. I'll reply the thinking
(based on your approach below) on that thread later.

> I'm really tired of this tinkering frenzy. There is only one correct approach to
> this:

> 
>    1) Define the requirements
> 
>    2) Define the best trapping mechanism
> 
>    3) Sit down, look at the existing code including the FPU rework for
>       AMX. Come up with a proper integration plan
> 
>    4) Clean up the existing KVM FPU mess further so the integration
>       can be done smoothly
> 
>    5) Add the required infrastructure in FPU core and KVM
> 
>    6) Add the trapping mechanics
> 
>    7) Enable feature
> 
> What you are doing is looking for the quickest way to duct tape this into the
> existing mess.
> 
> That might be matching the KVM expectations, but it's not going to happen.
> 
> KVM already violates all well known rules of encapsulation and just fiddles in
> the guts of FPU mechanism, duplicates code in buggy ways.
> 
> This has to stop now!
> 

Yes, this is an opportunity to make current KVM FPU better.  

> You are free to ignore me,
Of course I won't, because I also want to try a good way that both KVM 
and kernel are glad to use.  

Thanks,
Jing

 but all you are going to achieve is to delay AMX
> integration further. Seriously, I'm not even going to reply to anything which is
> not based on the above approach.
> 
> I'm sure you can figure out at which point we are at the moment.
> 
> Thanks,
> 
>         tglx