On Wed, May 27, 2015 at 08:32:04AM +0300, Avi Kivity wrote:
> On 05/27/2015 05:06 AM, Steve Rutherford wrote:
> >On Sun, May 24, 2015 at 07:46:03PM +0300, Avi Kivity wrote:
> >>On 05/13/2015 04:47 AM, Steve Rutherford wrote:
> >>>Adds KVM_EXIT_IOAPIC_EOI which passes the interrupt vector up to
> >>>userspace.
> >>>
> >>>Uses a per VCPU exit bitmap to decide whether or not the IOAPIC needs
> >>>to be informed (which is identical to the EOI_EXIT_BITMAP field used
> >>>by modern x86 processors, but can also be used to elide kvm IOAPIC EOI
> >>>exits on older processors).
> >>>
> >>>[Note: A prototype using ResampleFDs found that decoupling the EOI
> >>>from the VCPU's thread made it possible for the VCPU to not see a
> >>>recent EOI after reentering the guest. This does not match real
> >>>hardware.]
> >>>
> >>>Compile tested for Intel x86.
> >>>
> >>>Signed-off-by: Steve Rutherford <srutherford@xxxxxxxxxx>
> >>>---
> >>> Documentation/virtual/kvm/api.txt | 10 ++++++++++
> >>> arch/x86/include/asm/kvm_host.h | 3 +++
> >>> arch/x86/kvm/lapic.c | 9 +++++++++
> >>> arch/x86/kvm/x86.c | 11 +++++++++++
> >>> include/linux/kvm_host.h | 1 +
> >>> include/uapi/linux/kvm.h | 5 +++++
> >>> 6 files changed, 39 insertions(+)
> >>>
> >>>diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> >>>index 0744b4e..dd92996 100644
> >>>--- a/Documentation/virtual/kvm/api.txt
> >>>+++ b/Documentation/virtual/kvm/api.txt
> >>>@@ -3285,6 +3285,16 @@ Valid values for 'type' are:
> >>> */
> >>> __u64 kvm_valid_regs;
> >>> __u64 kvm_dirty_regs;
> >>>+
> >>>+ /* KVM_EXIT_IOAPIC_EOI */
> >>>+ struct {
> >>>+ __u8 vector;
> >>>+ } eoi;
> >>>+
> >>>+Indicates that an eoi of a level triggered IOAPIC interrupt on vector has
> >>>+occurred, which should be handled by the userspace IOAPIC. Triggers when
> >>>+the Irqchip has been split between userspace and the kernel.
> >>>+
> >>The ioapic is a global resource, so it doesn't make sense for
> >>information about it to be returned in a per-vcpu structure
> >EOI exits are a per-vcpu behavior, so this doesn't seem all that strange.
> >
> >>(or to block the vcpu while it is being processed).
> >Blocking doesn't feel clean, but doesn't seem all that bad, given
> >that these operations are relatively rare on modern configurations.
>
> Agree, maybe the realtime people have an interest here.
>
> >>The way I'd model it is to emulate the APIC bus that connects local
> >>APICs and the IOAPIC, using a socket pair. When the user-space
> >>ioapic wants to inject an interrupt, it sends a message to the local
> >>APICs which then inject it, and when it's ack'ed the EOI is sent
> >>back on the same bus.
> >Although I'm not certain about this, it sounds to me like this would
> >require a kernel thread to be waiting (in some way) on this socket, which
> >seems rather heavy handed.
>
> It's been a while since I did kernel programming, but I think you
> can queue a callback to be called when an I/O is ready, and not
> require a thread. IIRC we do that with irqfd to cause an interrupt
> to be injected.
>
This should be possible, but it's going to add a ton of complexity, and I don't really see any compelling benefits. If there is a compelling reason to switch to a socket based interface, I'm definitely willing to refactor.
Steve
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