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Re: [PATCH for-5.0] xive/kvm: Trigger interrupts from userspace


From: Cédric Le Goater
Subject: Re: [PATCH for-5.0] xive/kvm: Trigger interrupts from userspace
Date: Tue, 19 Nov 2019 09:15:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1

On 19/11/2019 01:47, David Gibson wrote:
> On Mon, Nov 18, 2019 at 04:37:16PM +0100, Cédric Le Goater wrote:
>> On 18/11/2019 16:12, Greg Kurz wrote:
>>> When using the XIVE KVM device, the trigger page is directly accessible
>>> in QEMU. Unlike with XICS, no need to ask KVM to fire the interrupt. A
>>> simple store on the trigger page does the job.
>>>
>>> Just call xive_esb_trigger().
>>
>> Yes but the KVM XIVE device does a few other checks. 
>>
>> It checks that the interrupt was correctly initialized at the KVM device
>> level. We should be fine in QEMU which has similar checks.
>>
>> It caches the LSI assertion level. We should be fine also because it is
>> useless in KVM when using the XIVE native exploitation mode.
>>
>> It checks it is not a passthru interrupt. Any idea on how to check this 
>> condition under QEMU ? 
>>  
>>> This may improve performance of emulated devices that go through
>>> qemu_set_irq(), eg. virtio devices created with ioeventfd=off or
>>> configured by the guest to use LSI interrupts, which aren't really
>>> recommended setups.
>>
>> LGTM.
> 
> Ok, between the comments above and this, I'm not sure if this is ready
> to merge or not.

I think it is. 

With this change, we are loosing a check on passthrough interrupts but 
I am not sure how critical this is given that QEMU can anyhow bypass 
KVM and trigger the interrupt using a store on the ESB page. 

>> Any figures to share ? 

I am torturing Greg to have numbers :) but he resisted well.

>> C.
>>
>>> Signed-off-by: Greg Kurz <address@hidden>

Let's move on.

Reviewed-by: Cédric Le Goater <address@hidden>

C.

>>> ---
>>>  hw/intc/spapr_xive_kvm.c |   16 ++--------------
>>>  1 file changed, 2 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/hw/intc/spapr_xive_kvm.c b/hw/intc/spapr_xive_kvm.c
>>> index 08012ac7cd76..69e73552f1ef 100644
>>> --- a/hw/intc/spapr_xive_kvm.c
>>> +++ b/hw/intc/spapr_xive_kvm.c
>>> @@ -354,32 +354,20 @@ static void kvmppc_xive_source_get_state(XiveSource 
>>> *xsrc)
>>>  void kvmppc_xive_source_set_irq(void *opaque, int srcno, int val)
>>>  {
>>>      XiveSource *xsrc = opaque;
>>> -    SpaprXive *xive = SPAPR_XIVE(xsrc->xive);
>>> -    struct kvm_irq_level args;
>>> -    int rc;
>>> -
>>> -    /* The KVM XIVE device should be in use */
>>> -    assert(xive->fd != -1);
>>>  
>>> -    args.irq = srcno;
>>>      if (!xive_source_irq_is_lsi(xsrc, srcno)) {
>>>          if (!val) {
>>>              return;
>>>          }
>>> -        args.level = KVM_INTERRUPT_SET;
>>>      } else {
>>>          if (val) {
>>>              xsrc->status[srcno] |= XIVE_STATUS_ASSERTED;
>>> -            args.level = KVM_INTERRUPT_SET_LEVEL;
>>>          } else {
>>>              xsrc->status[srcno] &= ~XIVE_STATUS_ASSERTED;
>>> -            args.level = KVM_INTERRUPT_UNSET;
>>>          }
>>>      }
>>> -    rc = kvm_vm_ioctl(kvm_state, KVM_IRQ_LINE, &args);
>>> -    if (rc < 0) {
>>> -        error_report("XIVE: kvm_irq_line() failed : %s", strerror(errno));
>>> -    }
>>> +
>>> +    xive_esb_trigger(xsrc, srcno);
>>>  }
>>
>>
> 




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