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

[Avi Kivity <avi.kivity@gmail.com>]

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 (or to block 
the vcpu while it is being processed).

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.

It's true that the APIC bus no longer exists, but modern processors 
still pretend it does.