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From: Thomas Gleixner <tglx@linutronix.de>
To: Paolo Bonzini <pbonzini@redhat.com>,
	"Liu, Jing2" <jing2.liu@intel.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 11:36:25 +0200	[thread overview]
Message-ID: <87a6ja6352.ffs@tglx> (raw)
In-Reply-To: <87lf2v5shb.ffs@tglx>

Paolo,

On Thu, Oct 14 2021 at 21:14, Thomas Gleixner wrote:
> On Thu, Oct 14 2021 at 17:01, Paolo Bonzini wrote:
>>> vcpu_create()
>>> 
>>>    fpu_init_fpstate_user(guest_fpu, supported_xcr0)
>>> 
>>> That will (it does not today) do:
>>> 
>>>       guest_fpu::__state_perm = supported_xcr0 & xstate_get_group_perm();
>>> 
>>> The you have the information you need right in the guest FPU.
>>
>> Good, I wasn't aware of the APIs that will be there.
>
> Me neither, but that's a pretty obvious consequence of the work I'm
> doing for AMX. So I made it up for you. :)

let me make some more up for you!

If you carefully look at part 2 of the rework, then you might notice
that there is a fundamental change which allows to do a real
simplification for KVM FPU handling:

   current->thread.fpu.fpstate

is now a pointer. So you can spare one FPU allocation because we can now
do:

fpu_attach_guest_fpu(supported_xcr0)
{
        guest_fpstate = alloc_fpstate(supported_xcr0);
        fpu_init_fpstate_user(guest_fpstate, supported_xcr0);
        current->thread.fpu.guest_fpstate = guest_fpstate;
}

fpu_swap_kvm_fpu() becomes in the first step:

fpu_swap_kvm_fpu(bool enter_guest)
{
        safe_fpregs_to_fpstate(current->thread.fpu.fpstate);

        swap(current->thread.fpu.fpstate, current->thread.fpu.guest_fpstate);

        restore_fpregs_from_fpstate(current->thread.fpu.fpstate);
}

@enter guest will allow to do some sanity checks

In a second step:

fpu_swap_kvm_fpu(bool enter_guest, u64 guest_needs_features)
{
        possibly_reallocate(enter_guest, guest_needs_features);
        safe_fpregs_to_fpstate(current->thread.fpu.fpstate);

        swap(current->thread.fpu.fpstate, current->thread.fpu.guest_fpstate);

        restore_fpregs_from_fpstate(current->thread.fpu.fpstate);
        possibly_reallocate(enter_guest, guest_needs_features);
}

@guest_needs_features is the information which you gather via guest XCR0
and guest XFD.

So fpu_swap_kvm_fpu() is going to be the place where reallocation happens
and that's good enough for both cases:

vcpu_run()

     fpu_swap_kvm_fpu(); <- 1

     while (...)
           vmenter();

     fpu_swap_kvm_fpu(); <- 2

#1 QEMU user space used feature and has already large fpstate

#2 Guest requires feature but has not used it yet (XCR0/XFD trapping)

See?

It's not only correct, it's also simple and truly beautiful.

Thanks,

        tglx

  parent reply	other threads:[~2021-10-15  9:36 UTC|newest]

Thread overview: 96+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-11 23:59 [patch 00/31] x86/fpu: Preparatory cleanups for AMX support (part 1) Thomas Gleixner
2021-10-11 23:59 ` [patch 01/31] x86/fpu: Remove pointless argument from switch_fpu_finish() Thomas Gleixner
2021-10-12  0:00 ` [patch 02/31] x86/fpu: Update stale comments Thomas Gleixner
2021-10-12  0:00 ` [patch 03/31] x86/pkru: Remove useless include Thomas Gleixner
2021-10-12  0:00 ` [patch 04/31] x86/fpu: Restrict xsaves()/xrstors() to independent states Thomas Gleixner
2021-10-12 14:24   ` Borislav Petkov
2021-10-12  0:00 ` [patch 05/31] x86/fpu: Cleanup the on_boot_cpu clutter Thomas Gleixner
2021-10-12  0:00 ` [patch 06/31] x86/fpu: Remove pointless memset in fpu_clone() Thomas Gleixner
2021-10-12  0:00 ` [patch 07/31] x86/process: Clone FPU in copy_thread() Thomas Gleixner
2021-10-12  0:00 ` [patch 08/31] x86/fpu: Do not inherit FPU context for kernel and IO worker threads Thomas Gleixner
2021-10-12  0:00 ` [patch 09/31] x86/fpu: Do not inherit FPU context for CLONE_THREAD Thomas Gleixner
2021-10-12 16:10   ` Borislav Petkov
2021-10-12 18:52     ` Thomas Gleixner
2021-10-12 19:01       ` Thomas Gleixner
2021-10-12  0:00 ` [patch 10/31] x86/fpu: Cleanup xstate xcomp_bv initialization Thomas Gleixner
2021-10-12  0:00 ` [patch 11/31] x86/fpu/xstate: Provide and use for_each_xfeature() Thomas Gleixner
2021-10-12 16:45   ` Borislav Petkov
2021-10-12  0:00 ` [patch 12/31] x86/fpu/xstate: Mark all init only functions __init Thomas Gleixner
2021-10-12  0:00 ` [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core Thomas Gleixner
2021-10-12 16:53   ` Borislav Petkov
2021-10-12 18:25     ` Thomas Gleixner
2021-10-12 18:26       ` Thomas Gleixner
2021-10-12 17:22   ` Paolo Bonzini
2021-10-13  6:15     ` Liu, Jing2
2021-10-13  6:26       ` Paolo Bonzini
2021-10-13  7:46         ` Liu, Jing2
2021-10-13  8:42           ` Paolo Bonzini
2021-10-13 10:14             ` Andy Lutomirski
2021-10-13 12:26               ` Paolo Bonzini
2021-10-13 14:14                 ` Thomas Gleixner
2021-10-13 14:24                   ` Thomas Gleixner
2021-10-13 14:59                 ` Andy Lutomirski
2021-10-13 15:05                   ` Paolo Bonzini
2021-10-13 10:25             ` Liu, Jing2
2021-10-13 12:37               ` Paolo Bonzini
2021-10-13 14:06             ` Thomas Gleixner
2021-10-14  6:50               ` Paolo Bonzini
2021-10-14  8:02                 ` Liu, Jing2
2021-10-14  9:01                   ` Paolo Bonzini
2021-10-14 11:21                     ` Liu, Jing2
2021-10-14 11:33                       ` Paolo Bonzini
2021-10-14 11:30                     ` Liu, Jing2
2021-10-14 11:39                       ` Paolo Bonzini
2021-11-22  8:50                         ` Liu, Jing2
2021-10-14 14:09                     ` Thomas Gleixner
2021-10-14 14:37                       ` Thomas Gleixner
2021-10-14 15:01                       ` Paolo Bonzini
2021-10-14 19:14                         ` Thomas Gleixner
2021-10-15  9:20                           ` Liu, Jing2
2021-10-15  9:36                           ` Thomas Gleixner [this message]
2021-10-15 14:24                             ` Liu, Jing2
2021-10-15 15:53                               ` Paolo Bonzini
2021-10-16 14:45                               ` Thomas Gleixner
2021-10-15  9:00                         ` Liu, Jing2
2021-10-15 10:50                           ` Thomas Gleixner
2021-10-15 11:17                             ` Paolo Bonzini
2021-10-15 13:01                             ` Liu, Jing2
2021-10-14 12:23                 ` Thomas Gleixner
2021-10-14 12:26                   ` Paolo Bonzini
2021-10-14 14:23                     ` Thomas Gleixner
2021-10-13 15:12       ` Thomas Gleixner
2021-10-14  8:21         ` Liu, Jing2
2021-10-14 13:08           ` Thomas Gleixner
2021-10-12  0:00 ` [patch 14/31] x86/fpu: Replace KVMs homebrewn FPU copy from user Thomas Gleixner
2021-10-12 17:00   ` Borislav Petkov
2021-10-13 14:57     ` Sean Christopherson
2021-10-13 15:12       ` Paolo Bonzini
2021-10-13 15:16       ` Thomas Gleixner
2021-10-12 17:30   ` Paolo Bonzini
2021-10-12  0:00 ` [patch 15/31] x86/fpu: Rework copy_xstate_to_uabi_buf() Thomas Gleixner
2021-10-12 17:30   ` Paolo Bonzini
2021-10-12  0:00 ` [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user Thomas Gleixner
2021-10-12 17:10   ` Borislav Petkov
2021-10-12 17:36   ` Paolo Bonzini
2021-10-12 17:47     ` Thomas Gleixner
2021-10-12 18:40       ` [patch V2 16/31] x86/fpu: Replace KVMs home brewed " Thomas Gleixner
2021-10-13  5:34       ` [patch 16/31] x86/fpu: Replace KVMs homebrewn " Paolo Bonzini
2021-10-12  0:00 ` [patch 17/31] x86/fpu: Mark fpu__init_prepare_fx_sw_frame() as __init Thomas Gleixner
2021-10-12  0:00 ` [patch 18/31] x86/fpu: Move context switch and exit to user inlines into sched.h Thomas Gleixner
2021-10-12  0:00 ` [patch 19/31] x86/fpu: Clean up cpu feature tests Thomas Gleixner
2021-10-12  0:00 ` [patch 20/31] x86/fpu: Make os_xrstor_booting() private Thomas Gleixner
2021-10-12  0:00 ` [patch 21/31] x86/fpu: Move os_xsave() and os_xrstor() to core Thomas Gleixner
2021-10-12  0:00 ` [patch 22/31] x86/fpu: Move legacy ASM wrappers " Thomas Gleixner
2021-10-12  0:00 ` [patch 23/31] x86/fpu: Make WARN_ON_FPU() private Thomas Gleixner
2021-10-12  0:00 ` [patch 24/31] x86/fpu: Move fpregs_restore_userregs() to core Thomas Gleixner
2021-10-12 17:32   ` Borislav Petkov
2021-10-12  0:00 ` [patch 25/31] x86/fpu: Move mxcsr related code " Thomas Gleixner
2021-10-12  0:00 ` [patch 26/31] x86/fpu: Move fpstate functions to api.h Thomas Gleixner
2021-10-12 17:46   ` Borislav Petkov
2021-10-12  0:00 ` [patch 27/31] x86/fpu: Remove internal.h dependency from fpu/signal.h Thomas Gleixner
2021-10-12  0:00 ` [patch 28/31] x86/sev: Include fpu/xcr.h Thomas Gleixner
2021-10-12  7:24   ` Xiaoyao Li
2021-10-12  0:00 ` [patch 29/31] x86/fpu: Mop up the internal.h leftovers Thomas Gleixner
2021-10-12  0:00 ` [patch 30/31] x86/fpu: Replace the includes of fpu/internal.h Thomas Gleixner
2021-10-12  0:00 ` [patch 31/31] x86/fpu: Provide a proper function for ex_handler_fprestore() Thomas Gleixner
2021-10-12 21:15 ` [patch 00/31] x86/fpu: Preparatory cleanups for AMX support (part 1) Thomas Gleixner

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