Re: [RFC PATCH 2/4] KVM: x86: Add KVM exit for IOAPIC EOIs

[Steve Rutherford <srutherford@google.com>]

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@google.com>
> >---
> >  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.

> 
> 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.