On Thu, Sep 22, 2011 at 01:52:56PM +0300, Nadav Har'El wrote:
> This patch adds a new vcpu->requests bit, KVM_REQ_IMMEDIATE_EXIT.
> This bit requests that when next entering the guest, we should run it only
> for as little as possible, and exit again.
>
> We use this new option in nested VMX: When L1 launches L2, but L0 wishes L1
> to continue running so it can inject an event to it, we unfortunately cannot
> just pretend to have run L2 for a little while - We must really launch L2,
> otherwise certain one-off vmcs12 parameters (namely, L1 injection into L2)
> will be lost. So the existing code runs L2 in this case.
> But L2 could potentially run for a long time until it exits, and the
> injection into L1 will be delayed. The new KVM_REQ_IMMEDIATE_EXIT allows us
> to request that L2 will be entered, as necessary, but will exit as soon as
> possible after entry.
>
> Our implementation of this request uses smp_send_reschedule() to send a
> self-IPI, with interrupts disabled. The interrupts remain disabled until the
> guest is entered, and then, after the entry is complete (often including
> processing an injection and jumping to the relevant handler), the physical
> interrupt is noticed and causes an exit.
>
> On recent Intel processors, we could have achieved the same goal by using
> MTF instead of a self-IPI. Another technique worth considering in the future
> is to use VM_EXIT_ACK_INTR_ON_EXIT and a highest-priority vector IPI - to
> slightly improve performance by avoiding the useless interrupt handler
> which ends up being called when smp_send_reschedule() is used.
>
> Signed-off-by: Nadav Har'El <nyh@xxxxxxxxxx>
> ---
> arch/x86/kvm/vmx.c | 11 +++++++----
> arch/x86/kvm/x86.c | 6 ++++++
> include/linux/kvm_host.h | 1 +
> 3 files changed, 14 insertions(+), 4 deletions(-)
>
> --- .before/include/linux/kvm_host.h 2011-09-22 13:51:31.000000000 +0300
> +++ .after/include/linux/kvm_host.h 2011-09-22 13:51:31.000000000 +0300
> @@ -48,6 +48,7 @@
> #define KVM_REQ_EVENT 11
> #define KVM_REQ_APF_HALT 12
> #define KVM_REQ_STEAL_UPDATE 13
> +#define KVM_REQ_IMMEDIATE_EXIT 14
>
> #define KVM_USERSPACE_IRQ_SOURCE_ID 0
>
> --- .before/arch/x86/kvm/x86.c 2011-09-22 13:51:31.000000000 +0300
> +++ .after/arch/x86/kvm/x86.c 2011-09-22 13:51:31.000000000 +0300
> @@ -5610,6 +5610,7 @@ static int vcpu_enter_guest(struct kvm_v
> bool nmi_pending;
> bool req_int_win = !irqchip_in_kernel(vcpu->kvm) &&
> vcpu->run->request_interrupt_window;
> + bool req_immediate_exit = 0;
>
> if (vcpu->requests) {
> if (kvm_check_request(KVM_REQ_MMU_RELOAD, vcpu))
> @@ -5647,6 +5648,8 @@ static int vcpu_enter_guest(struct kvm_v
> }
> if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
> record_steal_time(vcpu);
> + req_immediate_exit =
> + kvm_check_request(KVM_REQ_IMMEDIATE_EXIT, vcpu);
The immediate exit information can be lost if entry decides to bail out.
You can do
req_immediate_exit = kvm_check_request(KVM_REQ_IMMEDIATE_EXIT)
after preempt_disable()
and then transfer back the bit in the bail out case in
if (vcpu->mode == EXITING_GUEST_MODE || vcpu->requests
...
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