Re: [Qemu-devel] [PATCH 2/2] intel-iommu: restrict EIM to quirkless KVM

[Peter Xu]

On Wed, Aug 10, 2016 at 06:59:14PM +0200, Radim Krčmář wrote:
> 2016-08-10 11:29+0800, Peter Xu:
> > On Tue, Aug 09, 2016 at 05:03:33PM +0200, Radim Krčmář wrote:
> >> APIC in QEMU doesn't support x2APIC so exposing EIM is pointless and KVM
> >> has a quirk that needs to be disabled unless we want x2APIC message with
> >> destination 0xff to be misinterpreted as a broadcast.
> >> 
> >> Signed-off-by: Radim Krčmář <address@hidden>
> >> ---
> >> diff --git a/hw/i386/intel_iommu.c b/hw/i386/intel_iommu.c
> >> @@ -31,6 +31,7 @@
> >>  #include "hw/i386/x86-iommu.h"
> >>  #include "hw/pci-host/q35.h"
> >>  #include "sysemu/kvm.h"
> >> +#include "kvm_i386.h"
> >>  
> >>  /*#define DEBUG_INTEL_IOMMU*/
> >>  #ifdef DEBUG_INTEL_IOMMU
> >> @@ -2364,7 +2365,14 @@ static void vtd_init(IntelIOMMUState *s)
> >>      s->ecap = VTD_ECAP_QI | VTD_ECAP_IRO;
> >>  
> >>      if (x86_iommu->intr_supported) {
> >> -        s->ecap |= VTD_ECAP_IR | VTD_ECAP_EIM | VTD_ECAP_MHMV;
> >> +        s->ecap |= VTD_ECAP_IR | VTD_ECAP_MHMV;
> >> +        /* QEMU APIC does not support x2APIC and KVM does not work well 
> >> without
> >> +         * disabling a quirk.  IOMMU is unmigratable so we 
> >> unconditionally use
> >> +         * optional KVM features.
> >> +         */
> >> +        if (kvm_irqchip_in_kernel() && 
> >> kvm_disable_x2apic_broadcast_quirk()) {
> >> +            s->ecap |= VTD_ECAP_EIM;
> >> +        }
> > 
> > Good to me if this patch is only going to disable x2apic when we
> > failed to disable the x2apic broadcast quirk in KVM.
> 
> Do you mean to also allow QEMU's APIC?
> 
>   if (!kvm_irqchip_in_kernel() || kvm_disable_x2apic_broadcast_quirk())
> 
> Thanks.

Nop. But, now I understand your mean, and please take this:

Reviewed-by: Peter Xu <address@hidden>

> 
> > Question: still not too clear about how KVM treats the case when
> > x2apic and xapic are used in a single VM. E.g., if dest_id of an
> > interrupt is 0xff from a peripheral device, how should I know this is
> > a x2apic broadcast to 0-7 cpu in cluster 0, or an apic broadcast to
> > all?
> 
> If a KVM guest has LAPICs in both x and x2 modes, then every interrupt
> arrives to all LAPICs and is accepted according to ID/LDR/DFR where
> every LAPIC assumes that the sender matches LAPIC's mode => all xLAPICs
> would accept 0xff and x2LAPICs with ID 0-7 would as well.
> kvm_apic_match_dest() is the function that decides and kvm_apic_mda()
> does most of the magic.  The quirk disables a case that translated 0xff
> to 0xffffffff for x2LAPICs.
> 
> I don't know how real hardware does it and the behavior might even
> differ between FSB and QPI.  I think KVM differs from both of them, but
> it's not that any behavior makes a difference in practice, so running a
> test kernel to figure it out has never been a priority ...

Thank you for the explaination!

I feel I am getting close to the nightmares. :)

-- peterx