Date:   Wed, 4 Oct 2017 22:16:55 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH v2 4/4] KVM: LAPIC: Don't silently accept bad vectors

2017-10-04 20:01 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> 2017-10-04 15:56+0800, Wanpeng Li:
>> 2017-10-04 1:53 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> > 2017-09-28 18:04-0700, Wanpeng Li:
>> >> From: Wanpeng Li <wanpeng.li@...mail.com>
>> >>
>> >> Vectors 0-15 are reserved, and a physical LAPIC - upon sending or
>> >> receiving one - would generate an APIC error instead of doing the
>> >> requested action. Make our emulation behave similarly.
>> >>
>> >> Cc: Paolo Bonzini <pbonzini@...hat.com>
>> >> Cc: Radim Krčmář <rkrcmar@...hat.com>
>> >> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
>> >> ---
>> >>  arch/x86/kvm/lapic.c | 30 ++++++++++++++++++++++++++++--
>> >>  1 file changed, 28 insertions(+), 2 deletions(-)
>> >>
>> >> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> >> index 6bafd06..a779ba9 100644
>> >> --- a/arch/x86/kvm/lapic.c
>> >> +++ b/arch/x86/kvm/lapic.c
>> >> @@ -935,6 +935,25 @@ bool kvm_intr_is_single_vcpu_fast(struct kvm *kvm, struct kvm_lapic_irq *irq,
>> >>       return ret;
>> >>  }
>> >>
>> >> +static void apic_error(struct kvm_lapic *apic, unsigned long errmask)
>> >> +{
>> >> +     uint32_t esr;
>> >> +
>> >> +     esr = kvm_lapic_get_reg(apic, APIC_ESR);
>> >> +
>> >> +     if ((esr & errmask) != errmask) {
>> >
>> > The spec makes me think that there is going to be only 1 interrupt
>> > (regardless of the number errors) until the software writes 0 to
>> > APIC_ESR.  Is there a better description than the following 10.5.3?
>> >
>> >   The ESR is a write/read register. Before attempt to read from the ESR,
>> >   software should first write to it. (The value written does not affect
>> >   the values read subsequently; only zero may be written in x2APIC
>> >   mode.) This write clears any previously logged errors and updates the
>> >   ESR with any errors detected since the last write to the ESR. This
>> >   write also rearms the APIC error interrupt triggering mechanism.
>> >
>> > This also describes a different handling of APIC_ESR -- APIC_ESR is
>> > updated only on software writes to APIC_ESR.  All errors in between seem
>> > to be logged internally (not sure where to migrate it).
>>
>> Is there any thing need to be changed in this function?
>
> Yes.  For the first part, it should really be tested on bare-metal and
> modelled upon that.  SDM mentions some kind of rearming and APM doesn't
> so we maybe could just send the interrupt every time (if unmasked).
> And maybe vectors from external interrupts trigger the error too, but
> we definitely don't need to sort that out immediately.
>
> For the second part, the LAPIC error doesn't cause a write to APIC_ESR.
> We need to add a state for pending errors (and somehow migrate it) that
> gets copied to APIC_ESR after a write.
>
>> >> +             uint32_t lvterr = kvm_lapic_get_reg(apic, APIC_LVTERR);
>> >> +
>> >> +             kvm_lapic_set_reg(apic, APIC_ESR, esr | errmask);
>> >> +             if (!(lvterr & APIC_LVT_MASKED)) {
>> >> +                     struct kvm_lapic_irq irq;
>> >> +
>> >> +                     irq.vector = lvterr & 0xff;
>> >> +                     kvm_irq_delivery_to_apic(apic->vcpu->kvm, apic, &irq, NULL);
>> >> +             }
>> >> +     }
>> >> +}
>> >> +
>> >>  /*
>> >>   * Add a pending IRQ into lapic.
>> >>   * Return 1 if successfully added and 0 if discarded.
>> >> @@ -946,6 +965,11 @@ static int __apic_accept_irq(struct kvm_lapic *apic, int delivery_mode,
>> >>       int result = 0;
>> >>       struct kvm_vcpu *vcpu = apic->vcpu;
>> >>
>> >> +     if (unlikely(vector < 16) && delivery_mode == APIC_DM_FIXED) {
>> >> +             apic_error(apic, APIC_ESR_RECVILL);
>> >
>> > The error is also triggered if lowest priority is supported and tries to
>> > deliver an invalid vector.
>>
>> Could you point out this in SDM? :)
>
> In section 10.5.3 Error Handling:
>
>   If the local APIC does not support the sending of lowest-priority IPIs
>   and software writes the ICR to send a lowest-priority IPI with an
>   illegal vector, the local APIC sets only the “redirectible IPI” error
>   bit.
>
> Hence, if local APIC does support lowest-priority, then it throws the
> same error as fixed.  (KVM does support lowest-priority.)

Yeah, I read the section before but I misunderstand it. It seems that
the section means it just occurs when the local APIC does not support
the sending of lowest-priority IPIs?

Regards,
Wanpeng Li

>
>> >
>> >> +             return 0;
>> >> +     }
>> >> +
>> >>       trace_kvm_apic_accept_irq(vcpu->vcpu_id, delivery_mode,
>> >>                                 trig_mode, vector);
>> >>       switch (delivery_mode) {
>> >> @@ -1146,7 +1170,10 @@ static void apic_send_ipi(struct kvm_lapic *apic)
>> >>                  irq.trig_mode, irq.level, irq.dest_mode, irq.delivery_mode,
>> >>                  irq.vector, irq.msi_redir_hint);
>> >>
>> >> -     kvm_irq_delivery_to_apic(apic->vcpu->kvm, apic, &irq, NULL);
>> >> +     if (unlikely(irq.vector < 16 && irq.delivery_mode == APIC_DM_FIXED))
>> >
>> > Please check how APICv self-IPI acceleration behaves, so we're
>> > consistent.
>>
>> There is no vmexit for APICv self-IPI, so I think we can't intercept the vector.
>
> Right, so does it deliver the 0-15 vector?  If yes, then we should do
> that as well.  Otherwise, where does it save the error flag and does it
> send an error interrupt?  Or do we get a VM exit after all?
>
> Thanks.

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