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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	kvm@vger.kernel.org, Alex Williamson <alex.williamson@redhat.com>,
	Pei Zhang <pezhang@redhat.com>
Subject: Re: [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device
Date: Wed, 12 May 2021 08:10:10 -0300	[thread overview]
Message-ID: <20210512111010.GA232673@fuller.cnet> (raw)
In-Reply-To: <YJsjeEl80KzAXNFE@t490s>

On Tue, May 11, 2021 at 08:38:16PM -0400, Peter Xu wrote:
> On Tue, May 11, 2021 at 09:02:59PM -0300, Marcelo Tosatti wrote:
> > On Tue, May 11, 2021 at 08:51:24PM -0300, Marcelo Tosatti wrote:
> > > On Tue, May 11, 2021 at 05:35:41PM -0400, Peter Xu wrote:
> > > > On Tue, May 11, 2021 at 02:18:10PM -0300, Marcelo Tosatti wrote:
> > > > > On Tue, May 11, 2021 at 12:19:56PM -0400, Peter Xu wrote:
> > > > > > On Tue, May 11, 2021 at 11:51:57AM -0300, Marcelo Tosatti wrote:
> > > > > > > On Tue, May 11, 2021 at 10:39:11AM -0400, Peter Xu wrote:
> > > > > > > > On Fri, May 07, 2021 at 07:08:31PM -0300, Marcelo Tosatti wrote:
> > > > > > > > > > Wondering whether we should add a pi_test_on() check in kvm_vcpu_has_events()
> > > > > > > > > > somehow, so that even without customized ->vcpu_check_block we should be able
> > > > > > > > > > to break the block loop (as kvm_arch_vcpu_runnable will return true properly)?
> > > > > > > > > 
> > > > > > > > > static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
> > > > > > > > > {
> > > > > > > > >         int ret = -EINTR;
> > > > > > > > >         int idx = srcu_read_lock(&vcpu->kvm->srcu);
> > > > > > > > > 
> > > > > > > > >         if (kvm_arch_vcpu_runnable(vcpu)) {
> > > > > > > > >                 kvm_make_request(KVM_REQ_UNHALT, vcpu); <---
> > > > > > > > >                 goto out;
> > > > > > > > >         }
> > > > > > > > > 
> > > > > > > > > Don't want to unhalt the vcpu.
> > > > > > > > 
> > > > > > > > Could you elaborate?  It's not obvious to me why we can't do that if
> > > > > > > > pi_test_on() returns true..  we have pending post interrupts anyways, so
> > > > > > > > shouldn't we stop halting?  Thanks!
> > > > > > > 
> > > > > > > pi_test_on() only returns true when an interrupt is signalled by the
> > > > > > > device. But the sequence of events is:
> > > > > > > 
> > > > > > > 
> > > > > > > 1. pCPU idles without notification vector configured to wakeup vector.
> > > > > > > 
> > > > > > > 2. PCI device is hotplugged, assigned device count increases from 0 to 1.
> > > > > > > 
> > > > > > > <arbitrary amount of time>
> > > > > > > 
> > > > > > > 3. device generates interrupt, sets ON bit to true in the posted
> > > > > > > interrupt descriptor.
> > > > > > > 
> > > > > > > We want to exit kvm_vcpu_block after 2, but before 3 (where ON bit
> > > > > > > is not set).
> > > > > > 
> > > > > > Ah yes.. thanks.
> > > > > > 
> > > > > > Besides the current approach, I'm thinking maybe it'll be cleaner/less LOC to
> > > > > > define a KVM_REQ_UNBLOCK to replace the pre_block hook (in x86's kvm_host.h):
> > > > > > 
> > > > > > #define KVM_REQ_UNBLOCK			KVM_ARCH_REQ(31)
> > > > > > 
> > > > > > We can set it in vmx_pi_start_assignment(), then check+clear it in
> > > > > > kvm_vcpu_has_events() (or make it a bool in kvm_vcpu struct?).
> > > > > 
> > > > > Can't check it in kvm_vcpu_has_events() because that will set
> > > > > KVM_REQ_UNHALT (which we don't want).
> > > > 
> > > > I thought it was okay to break the guest HLT? 
> > > 
> > > Intel:
> > > 
> > > "HLT-HALT
> > > 
> > > Description
> > > 
> > > Stops instruction execution and places the processor in a HALT state. An enabled interrupt (including NMI and
> > > SMI), a debug exception, the BINIT# signal, the INIT# signal, or the RESET# signal will resume execution. If an
> > > interrupt (including NMI) is used to resume execution after a HLT instruction, the saved instruction pointer
> > > (CS:EIP) points to the instruction following the HLT instruction."
> > > 
> > > AMD:
> > > 
> > > "6.5 Processor Halt
> > > The processor halt instruction (HLT) halts instruction execution, leaving the processor in the halt state.
> > > No registers or machine state are modified as a result of executing the HLT instruction. The processor
> > > remains in the halt state until one of the following occurs:
> > > • A non-maskable interrupt (NMI).
> > > • An enabled, maskable interrupt (INTR).
> > > • Processor reset (RESET).
> > > • Processor initialization (INIT).
> > > • System-management interrupt (SMI)."
> > > 
> > > The KVM_REQ_UNBLOCK patch will resume execution even any such event
> > 
> > 						  even without any such event
> > 
> > > occuring. So the behaviour would be different from baremetal.
> > 
> 
> What if we move that kvm_check_request() into kvm_vcpu_check_block()?
> 
> ---8<---
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 739e1bd59e8a9..e6fee59b5dab6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11177,9 +11177,6 @@ static inline bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
>              static_call(kvm_x86_smi_allowed)(vcpu, false)))
>                 return true;
>  
> -       if (kvm_check_request(KVM_REQ_UNBLOCK, vcpu))
> -               return true;
> -
>         if (kvm_arch_interrupt_allowed(vcpu) &&
>             (kvm_cpu_has_interrupt(vcpu) ||
>             kvm_guest_apic_has_interrupt(vcpu)))
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f68035355c08a..fc5f6bffff7fc 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -2925,6 +2925,10 @@ static int kvm_vcpu_check_block(struct kvm_vcpu *vcpu)
>                 kvm_make_request(KVM_REQ_UNHALT, vcpu);
>                 goto out;
>         }
> +#ifdef CONFIG_X86
> +       if (kvm_check_request(KVM_REQ_UNBLOCK, vcpu))
> +               return true;
> +#endif
>         if (kvm_cpu_has_pending_timer(vcpu))
>                 goto out;
>         if (signal_pending(current))
> ---8<---
> 
> (The CONFIG_X86 is ugly indeed.. but just to show what I meant, e.g. it can be
>  a boolean too I think)
> 
> Would this work?

That would work: but vcpu->requests are nicely checked (and processed) 
at vcpu_enter_guest, before guest entry. The proposed request does not 
follow that pattern.


  reply	other threads:[~2021-05-12 11:59 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-07 13:06 [patch 0/4] VMX: configure posted interrupt " Marcelo Tosatti
2021-05-07 13:06 ` [patch 1/4] KVM: x86: add start_assignment hook to kvm_x86_ops Marcelo Tosatti
2021-05-07 19:16   ` Peter Xu
2021-05-10 17:53     ` Marcelo Tosatti
2021-05-07 13:06 ` [patch 2/4] KVM: add arch specific vcpu_check_block callback Marcelo Tosatti
2021-05-07 13:06 ` [patch 3/4] KVM: x86: implement kvm_arch_vcpu_check_block callback Marcelo Tosatti
2021-05-07 13:06 ` [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device Marcelo Tosatti
2021-05-07 17:22   ` Sean Christopherson
2021-05-07 19:29     ` Peter Xu
2021-05-07 22:08       ` Marcelo Tosatti
2021-05-11 14:39         ` Peter Xu
2021-05-11 14:51           ` Marcelo Tosatti
2021-05-11 16:19             ` Peter Xu
2021-05-11 17:18               ` Marcelo Tosatti
2021-05-11 21:35                 ` Peter Xu
2021-05-11 23:51                   ` Marcelo Tosatti
2021-05-12  0:02                     ` Marcelo Tosatti
2021-05-12  0:38                       ` Peter Xu
2021-05-12 11:10                         ` Marcelo Tosatti [this message]
2021-05-12 14:41                       ` Sean Christopherson
2021-05-12 15:34                         ` Peter Xu
2021-05-10 17:26 [patch 0/4] VMX: configure posted interrupt descriptor when assigning device (v3) Marcelo Tosatti
2021-05-10 17:26 ` [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device Marcelo Tosatti
2021-05-24 15:55   ` Paolo Bonzini
2021-05-24 17:53     ` Marcelo Tosatti
2021-05-25 11:58       ` Paolo Bonzini
2021-05-11 23:57 [patch 0/4] VMX: configure posted interrupt descriptor when assigning device (v4) Marcelo Tosatti
2021-05-11 23:57 ` [patch 4/4] KVM: VMX: update vcpu posted-interrupt descriptor when assigning device Marcelo Tosatti

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