From: Paolo Bonzini <pbonzini@redhat.com>
To: "Liu, Jing2" <jing2.liu@intel.com>,
Thomas Gleixner <tglx@linutronix.de>,
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>
Subject: Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core
Date: Wed, 13 Oct 2021 10:42:53 +0200 [thread overview]
Message-ID: <da47ba42-b61e-d236-2c1c-9c5504e48091@redhat.com> (raw)
In-Reply-To: <BYAPR11MB325676AAA8A0785AF992A2B9A9B79@BYAPR11MB3256.namprd11.prod.outlook.com>
On 13/10/21 09:46, Liu, Jing2 wrote:
>
>> On 13/10/21 08:15, Liu, Jing2 wrote:
>>> After KVM passthrough XFD to guest, when vmexit opening irq window and
>>> KVM is interrupted, kernel softirq path can call
>>> kernel_fpu_begin() to touch xsave state. This function does XSAVES. If
>>> guest XFD[18] is 1, and with guest AMX state in register, then guest
>>> AMX state is lost by XSAVES.
>>
>> Yes, the host value of XFD (which is zero) has to be restored after vmexit.
>> See how KVM already handles SPEC_CTRL.
>
> I'm trying to understand why qemu's XFD is zero after kernel supports AMX.
There are three copies of XFD:
- the guest value stored in vcpu->arch.
- the "QEMU" value attached to host_fpu. This one only becomes zero if
QEMU requires AMX (which shouldn't happen).
- the internal KVM value attached to guest_fpu. When #NM happens, this
one becomes zero.
The CPU value is:
- the host_fpu value before kvm_load_guest_fpu and after
kvm_put_guest_fpu. This ensures that QEMU context switch is as cheap as
possible.
- the guest_fpu value between kvm_load_guest_fpu and kvm_put_guest_fpu.
This ensures that no state is lost in the case you are describing.
- the OR of the guest value and the guest_fpu value while the guest runs
(using either MSR load/save lists, or manual wrmsr like
pt_guest_enter/pt_guest_exit). This ensures that the host has the
opportunity to get a #NM exception, and allocate AMX state in the
guest_fpu and in current->thread.fpu.
> Yes, passthrough is done by two cases: one is guest #NM trapped;
> another is guest clearing XFD before it generates #NM (this is possible for
> guest), then passthrough.
> For the two cases, we passthrough and allocate buffer for guest_fpu, and
> current->thread.fpu.
I think it's simpler to always wait for #NM, it will only happen once
per vCPU. In other words, even if the guest clears XFD before it
generates #NM, the guest_fpu's XFD remains nonzero and an #NM vmexit is
possible. After #NM the guest_fpu's XFD is zero; then passthrough can
happen and the #NM vmexit trap can be disabled.
Paolo
next prev parent reply other threads:[~2021-10-13 8:43 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 [this message]
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
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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