linux-kernel - RE: [PATCH v8 03/13] KVM: Define a new interface kvm_intr_is_single

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Date:	Thu, 17 Sep 2015 23:15:21 +0000
From:	"Wu, Feng" <feng.wu@...el.com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	"alex.williamson@...hat.com" <alex.williamson@...hat.com>,
	"joro@...tes.org" <joro@...tes.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>
CC:	"eric.auger@...aro.org" <eric.auger@...aro.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Radim Kr?má? <rkrcmar@...hat.com>,
	"Wu, Feng" <feng.wu@...el.com>
Subject: RE: [PATCH v8 03/13] KVM: Define a new interface
 kvm_intr_is_single_vcpu()



> -----Original Message-----
> From: Paolo Bonzini [mailto:pbonzini@...hat.com]
> Sent: Thursday, September 17, 2015 10:25 PM
> To: Wu, Feng; alex.williamson@...hat.com; joro@...tes.org;
> mtosatti@...hat.com
> Cc: eric.auger@...aro.org; kvm@...r.kernel.org;
> iommu@...ts.linux-foundation.org; linux-kernel@...r.kernel.org; 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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