RE: [PATCH v8 03/13] KVM: Define a new interface kvm_intr_is_single_vcpu()

From: Wu, Feng
Date: Thu Sep 17 2015 - 19:15:31 EST




> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@xxxxxxxxxx]
> Sent: Thursday, September 17, 2015 10:25 PM
> To: Wu, Feng; alex.williamson@xxxxxxxxxx; joro@xxxxxxxxxx;
> mtosatti@xxxxxxxxxx
> Cc: eric.auger@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Radim
> Krčmář
> Subject: Re: [PATCH v8 03/13] KVM: Define a new interface
> kvm_intr_is_single_vcpu()
>
> >> On 17/09/2015 05:17, Wu, Feng wrote:
> >>>>>>> + if (irq->dest_mode == APIC_DEST_PHYSICAL) {
> >>>>>>> + if (irq->dest_id == 0xFF)
> >>>>>>> + goto out;
> >>>>>>> +
> >>>>>>> + if (irq->dest_id >= ARRAY_SIZE(map->phys_map)) {
> >>>>>
> >>>>> Warning here is wrong, the guest can trigger it.
> >>> Could you please share more information about how the guest
> >>> triggers these conditions (including the following two), Thanks
> >>> a lot!
> >>
> >> irq->dest_id is a 16-bit value, so it can be > 255.
> >
> > Yes, irq->dest_id is defined as u32, but by looking the current KVM
> > code, seems desst_id is used as an u8 variable, even in x2apic mode
> > the dest_id will not beyond 255 (except for broadcast dest in in x2apic
> > mode). Correct me if I am wrong. Thanks a lot!
>
> Actually you're right, the MSI destination is only 8 bits. I was
> confused because of
>
> #define MSI_ADDR_DEST_ID_MASK 0x00ffff0
>
> in arch/x86/include/asm/msidef.h. But there may be a bug, see below...
>
> >>> + if (cid >= ARRAY_SIZE(map->logical_map)) {
> >>> + WARN_ON_ONCE(1);
> >>
> >> In x2apic mode irq->dest_id could have bits 12..15 set.
> >
> > cid is gotten from bit 16 ..31 of the ldr (in apic_logical_id()), and
> > in x2apic mode, ldr is constructed in kvm_apic_set_x2apic_id() as
> > below:
> >
> > u32 ldr = ((id >> 4) << 16) | (1 << (id & 0xf));
> >
> > So in fact, cid is (id >> 4), I cannot think of why cid can beyond 15.
>
> I think kvm_apic_match_logical_addr for MSI and IOAPIC interrupts is
> buggy in x2apic mode.
>
> It does:
>
> if (apic_x2apic_mode(apic))
> return ((logical_id >> 16) == (mda >> 16))
> && (logical_id & mda & 0xffff) != 0;
>
> But mda is only 8-bits for MSI and IOAPIC interrupts.
>
> Radim, should kvm_apic_mda also handle the !ipi && x2apic_mda && dest_id
> != APIC_BROADCAST case? It never triggers with Linux because it uses
> only the physical mode (that's not super-easy to see;
> ioapic_set_affinity looks for the RTEs in irq_data->chip_data and that
> is allocated with kzalloc).
>
> > Do I miss something here? Thanks!
>
> No, you were right.
>
> But still I think the WARNs are unnecessary; it is conceivable that some
> future chipset adds support for more than 8-bits in the dest_id.

No problem, I agree with it. Here I just want clarify some questions, thanks
for the elaboration!

Thanks,
Feng

>
> Paolo
>
> > Thanks,
> > Feng
> >
> >>
> >> Paolo
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