From: "Tian, Kevin" <kevin.tian@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>,
"Zhong, Yang" <yang.zhong@intel.com>,
"x86@kernel.org" <x86@kernel.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"mingo@redhat.com" <mingo@redhat.com>,
"bp@alien8.de" <bp@alien8.de>,
"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>
Cc: "seanjc@google.com" <seanjc@google.com>,
"Nakajima, Jun" <jun.nakajima@intel.com>,
"jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
"Liu, Jing2" <jing2.liu@intel.com>,
"Zhong, Yang" <yang.zhong@intel.com>
Subject: RE: [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly
Date: Sat, 11 Dec 2021 03:07:40 +0000 [thread overview]
Message-ID: <BN9PR11MB5276DF25E38EE7C4F4D29F288C729@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <87pmq4vw54.ffs@tglx>
> From: Thomas Gleixner <tglx@linutronix.de>
> Sent: Saturday, December 11, 2021 8:11 AM
>
> On Tue, Dec 07 2021 at 19:03, Yang Zhong wrote:
> > diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
> > index 5089f2e7dc22..9811dc98d550 100644
> > --- a/arch/x86/kernel/fpu/core.c
> > +++ b/arch/x86/kernel/fpu/core.c
> > @@ -238,6 +238,7 @@ bool fpu_alloc_guest_fpstate(struct fpu_guest
> *gfpu)
> > fpstate->is_guest = true;
> >
> > gfpu->fpstate = fpstate;
> > + gfpu->xfd_err = XFD_ERR_GUEST_DISABLED;
>
> This wants to be part of the previous patch, which introduces the field.
>
> > gfpu->user_xfeatures = fpu_user_cfg.default_features;
> > gfpu->user_perm = fpu_user_cfg.default_features;
> > fpu_init_guest_permissions(gfpu);
> > @@ -297,6 +298,7 @@ int fpu_swap_kvm_fpstate(struct fpu_guest
> *guest_fpu, bool enter_guest)
> > fpu->fpstate = guest_fps;
> > guest_fps->in_use = true;
> > } else {
> > + fpu_save_guest_xfd_err(guest_fpu);
>
> Hmm. See below.
>
> > guest_fps->in_use = false;
> > fpu->fpstate = fpu->__task_fpstate;
> > fpu->__task_fpstate = NULL;
> > @@ -4550,6 +4550,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > kvm_steal_time_set_preempted(vcpu);
> > srcu_read_unlock(&vcpu->kvm->srcu, idx);
> >
> > + if (vcpu->preempted)
> > + fpu_save_guest_xfd_err(&vcpu->arch.guest_fpu);
>
> I'm not really exited about the thought of an exception cause register
> in guest clobbered state.
>
> Aside of that I really have to ask the question why all this is needed?
>
> #NM in the guest is slow path, right? So why are you trying to optimize
> for it?
This is really good information. The current logic is obviously
based on the assumption that #NM is frequently triggered.
>
> The straight forward solution to this is:
>
> 1) Trap #NM and MSR_XFD_ERR write
and #NM vmexit handler should be called in kvm_x86_handle_exit_irqoff()
before preemption is enabled, otherwise there is still a small window
where MSR_XFD_ERR might be clobbered after preemption enable and
before #NM handler is actually called.
>
> 2) When the guest triggers #NM is takes an VMEXIT and the host
> does:
>
> rdmsrl(MSR_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>
> injects the #NM and goes on.
>
> 3) When the guest writes to MSR_XFD_ERR it takes an VMEXIT and
> the host does:
>
> vcpu->arch.guest_fpu.xfd_err = msrval;
> wrmsrl(MSR_XFD_ERR, msrval);
>
> and goes back.
>
> 4) Before entering the preemption disabled section of the VCPU loop
> do:
>
> if (vcpu->arch.guest_fpu.xfd_err)
> wrmsrl(MSR_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
>
> 5) Before leaving the preemption disabled section of the VCPU loop
> do:
>
> if (vcpu->arch.guest_fpu.xfd_err)
> wrmsrl(MSR_XFD_ERR, 0);
>
> It's really that simple and pretty much 0 overhead for the regular case.
Much cleaner.
>
> If the guest triggers #NM with a high frequency then taking the VMEXITs
> is the least of the problems. That's not a realistic use case, really.
>
> Hmm?
>
> Thanks,
>
> tglx
Thanks
Kevin
next prev parent reply other threads:[~2021-12-11 3:07 UTC|newest]
Thread overview: 80+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-08 0:03 [PATCH 00/19] AMX Support in KVM Yang Zhong
2021-12-08 0:03 ` [PATCH 01/19] x86/fpu: Extend prctl() with guest permissions Yang Zhong
2021-12-14 0:16 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 02/19] x86/fpu: Prepare KVM for dynamically enabled states Yang Zhong
2021-12-13 9:12 ` Paolo Bonzini
2021-12-13 12:00 ` Thomas Gleixner
2021-12-13 12:45 ` Paolo Bonzini
2021-12-13 19:50 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 03/19] kvm: x86: Fix xstate_required_size() to follow XSTATE alignment rule Yang Zhong
2021-12-08 0:03 ` [PATCH 04/19] kvm: x86: Check guest xstate permissions when KVM_SET_CPUID2 Yang Zhong
2021-12-08 0:03 ` [PATCH 05/19] x86/fpu: Move xfd initialization out of __fpstate_reset() to the callers Yang Zhong
2021-12-10 22:33 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 06/19] x86/fpu: Add reallocation mechanims for KVM Yang Zhong
2021-12-08 0:03 ` [PATCH 07/19] kvm: x86: Propagate fpstate reallocation error to userspace Yang Zhong
2021-12-10 15:44 ` Paolo Bonzini
2021-12-08 0:03 ` [PATCH 08/19] x86/fpu: Move xfd_update_state() to xstate.c and export symbol Yang Zhong
2021-12-10 22:44 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 09/19] kvm: x86: Prepare reallocation check Yang Zhong
2021-12-13 9:16 ` Paolo Bonzini
2021-12-14 7:06 ` Tian, Kevin
2021-12-14 10:16 ` Paolo Bonzini
2021-12-14 14:41 ` Liu, Jing2
2021-12-15 7:09 ` Tian, Kevin
2021-12-08 0:03 ` [PATCH 10/19] kvm: x86: Emulate WRMSR of guest IA32_XFD Yang Zhong
2021-12-10 16:02 ` Paolo Bonzini
2021-12-13 7:51 ` Liu, Jing2
2021-12-13 9:01 ` Paolo Bonzini
2021-12-14 10:26 ` Yang Zhong
2021-12-14 11:24 ` Paolo Bonzini
2021-12-10 23:09 ` Thomas Gleixner
2021-12-13 15:06 ` Paolo Bonzini
2021-12-13 19:45 ` Thomas Gleixner
2021-12-13 21:23 ` Thomas Gleixner
2021-12-14 7:16 ` Tian, Kevin
2021-12-08 0:03 ` [PATCH 11/19] kvm: x86: Check fpstate reallocation in XSETBV emulation Yang Zhong
2021-12-08 0:03 ` [PATCH 12/19] x86/fpu: Prepare KVM for bringing XFD state back in-sync Yang Zhong
2021-12-10 23:11 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 13/19] kvm: x86: Disable WRMSR interception for IA32_XFD on demand Yang Zhong
2021-12-08 7:23 ` Liu, Jing2
2021-12-08 0:03 ` [PATCH 14/19] x86/fpu: Prepare for KVM XFD_ERR handling Yang Zhong
2021-12-10 16:16 ` Paolo Bonzini
2021-12-10 23:20 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 15/19] kvm: x86: Save and restore guest XFD_ERR properly Yang Zhong
2021-12-10 16:23 ` Paolo Bonzini
2021-12-10 22:01 ` Paolo Bonzini
2021-12-12 13:10 ` Yang Zhong
2021-12-11 0:10 ` Thomas Gleixner
2021-12-11 1:31 ` Paolo Bonzini
2021-12-11 3:23 ` Tian, Kevin
2021-12-11 13:10 ` Thomas Gleixner
2021-12-11 3:07 ` Tian, Kevin [this message]
2021-12-11 13:29 ` Thomas Gleixner
2021-12-12 1:50 ` Tian, Kevin
2021-12-12 9:10 ` Paolo Bonzini
2021-12-08 0:03 ` [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl Yang Zhong
2021-12-10 16:25 ` Paolo Bonzini
2021-12-10 16:30 ` Paolo Bonzini
2021-12-10 22:13 ` Paolo Bonzini
2021-12-13 8:23 ` Wang, Wei W
2021-12-13 9:24 ` Paolo Bonzini
2021-12-14 6:06 ` Wang, Wei W
2021-12-14 6:18 ` Paolo Bonzini
2021-12-15 2:39 ` Wang, Wei W
2021-12-15 13:42 ` Paolo Bonzini
2021-12-16 8:25 ` Wang, Wei W
2021-12-16 10:28 ` Paolo Bonzini
2021-12-20 17:54 ` State Component 18 and Palette 1 (Re: [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl) Nakajima, Jun
2021-12-22 14:44 ` Paolo Bonzini
2021-12-22 23:47 ` Nakajima, Jun
2021-12-22 14:52 ` Dave Hansen
2021-12-22 23:51 ` Nakajima, Jun
2021-12-13 10:10 ` [PATCH 16/19] kvm: x86: Introduce KVM_{G|S}ET_XSAVE2 ioctl Thomas Gleixner
2021-12-13 10:43 ` Paolo Bonzini
2021-12-13 12:40 ` Thomas Gleixner
2021-12-08 0:03 ` [PATCH 17/19] docs: virt: api.rst: Document the new KVM_{G, S}ET_XSAVE2 ioctls Yang Zhong
2021-12-08 0:03 ` [PATCH 18/19] kvm: x86: AMX XCR0 support for guest Yang Zhong
2021-12-10 16:30 ` Paolo Bonzini
2021-12-08 0:03 ` [PATCH 19/19] kvm: x86: Add AMX CPUIDs support Yang Zhong
2021-12-10 21:52 ` Paolo Bonzini
2021-12-11 21:20 ` [PATCH 00/19] AMX Support in KVM Thomas Gleixner
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