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

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Message-ID: <E959C4978C3B6342920538CF579893F002718968@SHSMSX104.ccr.corp.intel.com>
Date:	Thu, 17 Sep 2015 13:36:08 +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>,
	"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 5:42 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
> 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!

> 
> > +		if (!kvm_apic_logical_map_valid(map)) {
> > +			WARN_ON_ONCE(1);
> 
> Here, the guest can trigger it by setting a few APICs in flat mode and
> others in cluster mode, for example.

Oh, right, the logical map works only when the destination mode of all
the vCPUs are the same.

> 
> > +		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.
Do I miss something here? Thanks!

Thanks,
Feng

> 
> Paolo
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