Re: [PATCH 1/2] nVMX: Add KVM_REQ_IMMEDIATE_EXIT

[Marcelo Tosatti <mtosatti@redhat.com>]

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@il.ibm.com>
> ---
>  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
...