On 2011-09-02 16:30, Sasha Levin wrote: > On Fri, 2011-09-02 at 16:25 +0200, Jan Kiszka wrote: >> On 2011-09-02 16:11, Sasha Levin wrote: >>> On Fri, 2011-09-02 at 16:00 +0200, Jan Kiszka wrote: >>>> On 2011-09-02 15:13, Sasha Levin wrote: >>>>> On Fri, 2011-09-02 at 14:11 +0200, Jan Kiszka wrote: >>>>>> On 2011-09-02 13:36, Jan Kiszka wrote: >>>>>>> On 2011-09-02 13:27, Jan Kiszka wrote: >>>>>>>> On 2011-09-02 09:48, Sasha Levin wrote: >>>>>>>>> The RH bit exists in the message address register (lower 32 bits of >>>>>>>>> the address). >>>>>>>>> >>>>>>>>> The bit indicates whether the message should go to the processor >>>>>>>>> which was >>>>>>>>> indicated in the destination ID bits, or whether it should go to the >>>>>>>>> processor running at the lowest priority. >>>>>>>>> >>>>>>>>> Cc: Avi Kivity <[email protected]> >>>>>>>>> Cc: Marcelo Tosatti <[email protected]> >>>>>>>>> Signed-off-by: Sasha Levin <[email protected]> >>>>>>>>> --- >>>>>>>>> virt/kvm/irq_comm.c | 17 ++++++++++++++++- >>>>>>>>> 1 files changed, 16 insertions(+), 1 deletions(-) >>>>>>>>> >>>>>>>>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c >>>>>>>>> index 9f614b4..0ba3a3d 100644 >>>>>>>>> --- a/virt/kvm/irq_comm.c >>>>>>>>> +++ b/virt/kvm/irq_comm.c >>>>>>>>> @@ -134,7 +134,22 @@ int kvm_set_msi(struct >>>>>>>>> kvm_kernel_irq_routing_entry *e, >>>>>>>>> irq.level = 1; >>>>>>>>> irq.shorthand = 0; >>>>>>>>> >>>>>>>>> - /* TODO Deal with RH bit of MSI message address */ >>>>>>>>> + /* >>>>>>>>> + * If the RH bit is set, we'll deliver to the processor running >>>>>>>>> + * at the lowest priority. >>>>>>>>> + */ >>>>>>>>> + if (e->msi.address_lo & MSI_ADDR_REDIRECTION_LOWPRI) { >>>>>>>>> + irq.delivery_mode = MSI_DATA_DELIVERY_LOWPRI; >>>>>>>>> + } else { >>>>>>>>> + /* >>>>>>>>> + * If the RH bit is not set, we'll deliver to the >>>>>>>>> specific >>>>>>>>> + * processor mentioned in destination ID, and ignore >>>>>>>>> the DM >>>>>>>>> + * bit. >>>>>>>>> + */ >>>>>>>>> + irq.dest_mode = MSI_ADDR_DEST_MODE_PHYSICAL; >>>>>>>>> + irq.delivery_mode = MSI_DATA_DELIVERY_FIXED; >>>>>>>>> + } >>>>>>>>> + >>>>>>>>> return kvm_irq_delivery_to_apic(kvm, NULL, &irq); >>>>>>>>> } >>>>>>>>> >>>>>>>> >>>>>>>> Do you happen have a kvm unit test for this? Or how did you validate >>>>>>>> the >>>>>>>> change? It doesn't look incorrect to me, I'd just like to check it QEMU >>>>>>>> as well which apparently already has the logic above but also some >>>>>>>> contradictory comment. >>>>>>> >>>>>>> Err, no, QEMU does not have this logic, it also ignores RH. >>>>>>> >>>>>>> But the above bits make "irq.delivery_mode = e->msi.data & 0x700" >>>>>>> pointless. And that strongly suggests something is still wrong. >>>>>> >>>>>> I tend to believe that this is what the spec tries to tell us: >>>>>> >>>>>> diff --git a/virt/kvm/irq_comm.c b/virt/kvm/irq_comm.c >>>>>> index 9f614b4..b72f77a 100644 >>>>>> --- a/virt/kvm/irq_comm.c >>>>>> +++ b/virt/kvm/irq_comm.c >>>>>> @@ -128,7 +128,8 @@ int kvm_set_msi(struct kvm_kernel_irq_routing_entry >>>>>> *e, >>>>>> MSI_ADDR_DEST_ID_MASK) >> >>>>>> MSI_ADDR_DEST_ID_SHIFT; >>>>>> irq.vector = (e->msi.data & >>>>>> MSI_DATA_VECTOR_MASK) >> MSI_DATA_VECTOR_SHIFT; >>>>>> - irq.dest_mode = (1 << MSI_ADDR_DEST_MODE_SHIFT) & >>>>>> e->msi.address_lo; >>>>>> + irq.dest_mode = ((e->msi.address_lo & >>>>>> MSI_ADDR_DEST_MODE_LOGICAL) && >>>>>> + (e->msi.address_lo & MSI_ADDR_REDIRECTION_LOWPRI)); >>>>>> irq.trig_mode = (1 << MSI_DATA_TRIGGER_SHIFT) & e->msi.data; >>>>>> irq.delivery_mode = e->msi.data & 0x700; >>>>>> irq.level = 1; >>>>>> >>>>>> ie. the DM flag is only relevant if RH is set, and RH==0 is equivalent >>>>>> to RH==1 && DH==0. >>>>> >>>>> Thing is, the spec specifically states that RH==1 should deliver to >>>>> lowest priority - even though it doesn't state whats the relationship >>>>> between delivery mode and RH bit. >>>> >>>> The spec says "When RH is 1 and the physical destination mode is used >>>> [DM=0], the Destination ID field must not be set to 0xFF; it must point >>>> to a processor that is present and enabled to receive the interrupt." >>>> >>> >>> When RH=1 and DM=0 yes, but what happens when RH=1 and DM=1? >> >> irq.dest_mode becomes non-zero, and kvm_apic_match_dest uses >> kvm_apic_match_logical_addr for filtering out possible target CPUs. >> >> Mmh, a remaining question is if kvm_irq_delivery_to_apic is then already >> doing the right thing, even for delivery_mode != APIC_DM_LOWEST. >> > > The missing part is that when RH=1 we must look for the lowest priority: > > "Redirection hint indication (RH) - This bit indicates whether the > message should be directed to the processor with the lowest interrupt > priority among processors that can receive the interrupt." > > So it's not enough to set dest_mode, we must also make sure that > delivery_mode is set to low prio when RH=1.
That's debatable. delivery_mode == APIC_DM_LOWEST includes this target selection, but also more. I have a bad feeling when we just overwrite delivery_mode as defined by the MSI data field instead of only patching kvm_irq_delivery_to_apic or kvm_is_dm_lowest_prio - if required. > >> Again my question to you: Did you observe unexpected behaviour with some >> real guests, or is this just based on code and spec study so far? If we >> had a test case, that could also provide valuable hints. > > Sorry, no test case. > > I've stumbled on the 'TODO' comment when I was digging into the MSI > implementation in KVM and decided to implement it based on specs. Then we definitely need some blessing by Intel to avoid subtle regressions. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html
