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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 14:37:12 +0200	[thread overview]
Message-ID: <826be724-c92b-4310-7ee9-b0af7cf28844@redhat.com> (raw)
In-Reply-To: <BYAPR11MB325670EF536F5C4766C18D19A9B79@BYAPR11MB3256.namprd11.prod.outlook.com>

On 13/10/21 12:25, Liu, Jing2 wrote:
> [...]
>> - the internal KVM value attached to guest_fpu.  When #NM happens, this
>> one becomes zero.
> 
>> The CPU value is:
>>
>> - 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.
>>
> 
> OK, you mean using guest_fpu as a KVM value. Let me describe the
> flow to see if anything missing.
> 
> When #NM trap which makes passthrough, guest_fpu XFD set to 0 and keeps
> forever. (don't change HW XFD which is still 1)
> In the #NM trap, KVM alloc buffer and regenerate a #NM exception to guest
> to make guest kernel alloc its thread buffer.
> Then in next vmexit, KVM sync vcpu->arch.xfd, load guest_fpu value (=0) and
> update current->thread.fpu XFD to 0 for kernel reference.

In the #NM handler, KVM allocates the buffer and the guest_fpu XFD 
becomes zero.  Also because the guest_fpu XFD is zero:

- #NM vmexits are disabled.  More precisely, trapping #NM is only 
necessary if guest_fpu->xfd & ~vcpu->arch.xfd & vcpu->arch.xcr0 is 
nonzero (i.e. only if there is a state that is guest_fpu-disabled, but 
enabled according to both XFD and XCR0).

- On the next vmentry XFD is set to just vcpu->arch.xfd and the 
instruction is retried.  If the instruction causes an #NM in the guest, 
it is not trapped and delivered normally to the guest.

>> 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
> 
> You mean a wrmsr trap doesn't do anything and return back?

The guest might run with the same XFD value as before (which is 
guest_fpu->xfd | vcpu->arch.xfd), but vcpu->arch.xfd is changed.  The 
value in vcpu->arch.xfd will be read back by an RDMSR, because 
passthrough is not enabled and the RDMSR will cause a vmexit.

Once an #NM is received and guest_fpu->xfd becomes zero, passthrough can 
be enabled.

Paolo

> In this case, when next vmenter, the OR of the guest value
> (vcpu->arch.xfd) and the guest_fpu value is still 1, so this
> doesn't obey guest's HW assumption? (guest finds the wrmsr
> didn't work)


  reply	other threads:[~2021-10-13 12:37 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 [this message]
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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