Re: [PATCH v2] KVM: Implement support for the RH bit

Jan Kiszka
2011-09-02T12:11:28Z

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 <avi@redhat.com>
>>> Cc: Marcelo Tosatti <mtosatti@redhat.com>
>>> Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
>>> ---
>>> 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.

BTW, irq_comm.c is surely the wrong place for all this IA32-specific
interpretation of MSI address and data. And we have yet another
guest-triggerable printk in kvm_irq_delivery_to_apic (messages to
physical ID 0xff).