linux-kernel - Re: [PATCH v3 2/5] KVM: X86: Introduce need_cancel_enter

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Date:   Sun, 26 Apr 2020 10:05:00 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Haiwei Li <lihaiwei@...cent.com>
Subject: Re: [PATCH v3 2/5] KVM: X86: Introduce need_cancel_enter_guest helper

On Fri, 24 Apr 2020 at 14:23, Wanpeng Li <kernellwp@...il.com> wrote:
>
> From: Wanpeng Li <wanpengli@...cent.com>
>
> Introduce need_cancel_enter_guest() helper, we need to check some
> conditions before doing CONT_RUN, in addition, it can also catch
> the case vmexit occurred while another event was being delivered
> to guest software since vmx_complete_interrupts() adds the request
> bit.
>
> Tested-by: Haiwei Li <lihaiwei@...cent.com>
> Cc: Haiwei Li <lihaiwei@...cent.com>
> Signed-off-by: Wanpeng Li <wanpengli@...cent.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 12 +++++++-----
>  arch/x86/kvm/x86.c     | 10 ++++++++--
>  arch/x86/kvm/x86.h     |  1 +
>  3 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index f1f6638..5c21027 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6577,7 +6577,7 @@ bool __vmx_vcpu_run(struct vcpu_vmx *vmx, unsigned long *regs, bool launched);
>
>  static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
>  {
> -       enum exit_fastpath_completion exit_fastpath;
> +       enum exit_fastpath_completion exit_fastpath = EXIT_FASTPATH_NONE;
>         struct vcpu_vmx *vmx = to_vmx(vcpu);
>         unsigned long cr3, cr4;
>
> @@ -6754,10 +6754,12 @@ static enum exit_fastpath_completion vmx_vcpu_run(struct kvm_vcpu *vcpu)
>         vmx_recover_nmi_blocking(vmx);
>         vmx_complete_interrupts(vmx);
>
> -       exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
> -       /* static call is better with retpolines */
> -       if (exit_fastpath == EXIT_FASTPATH_CONT_RUN)
> -               goto cont_run;
> +       if (!kvm_need_cancel_enter_guest(vcpu)) {
> +               exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
> +               /* static call is better with retpolines */
> +               if (exit_fastpath == EXIT_FASTPATH_CONT_RUN)
> +                       goto cont_run;
> +       }

The kvm_need_cancel_enter_guest() should not before
vmx_exit_handlers_fastpath() which will break IPI fastpath. How about
applying something like below, otherwise, maybe introduce another
EXIT_FASTPATH_CONT_FAIL to indicate fails due to
kvm_need_cancel_enter_guest() if checking it after
vmx_exit_handlers_fastpath(), then we return 1 in vmx_handle_exit()
directly instead of kvm_skip_emulated_instruction(). VMX-preemption
timer exit doesn't need to skip emulated instruction but wrmsr
TSCDEADLINE MSR exit does which results in a little complex here.

Paolo, what do you think?

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 853d3af..9317924 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6564,6 +6564,9 @@ static enum exit_fastpath_completion
handle_fastpath_preemption_timer(struct kvm
 {
     struct vcpu_vmx *vmx = to_vmx(vcpu);

+    if (kvm_need_cancel_enter_guest(vcpu))
+        return EXIT_FASTPATH_NONE;
+
     if (!vmx->req_immediate_exit &&
         !unlikely(vmx->loaded_vmcs->hv_timer_soft_disabled)) {
             kvm_lapic_expired_hv_timer(vcpu);
@@ -6771,12 +6774,10 @@ static enum exit_fastpath_completion
vmx_vcpu_run(struct kvm_vcpu *vcpu)
     vmx_recover_nmi_blocking(vmx);
     vmx_complete_interrupts(vmx);

-    if (!(kvm_need_cancel_enter_guest(vcpu))) {
-        exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
-        if (exit_fastpath == EXIT_FASTPATH_CONT_RUN) {
-            vmx_sync_pir_to_irr(vcpu);
-            goto cont_run;
-        }
+    exit_fastpath = vmx_exit_handlers_fastpath(vcpu);
+    if (exit_fastpath == EXIT_FASTPATH_CONT_RUN) {
+        vmx_sync_pir_to_irr(vcpu);
+        goto cont_run;
     }

     return exit_fastpath;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 99061ba..11b309c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1618,6 +1618,9 @@ static int
handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data

 static int handle_fastpath_set_tscdeadline(struct kvm_vcpu *vcpu, u64 data)
 {
+    if (kvm_need_cancel_enter_guest(vcpu))
+        return 1;
+
     if (!kvm_x86_ops.set_hv_timer ||
         kvm_mwait_in_guest(vcpu->kvm) ||
         kvm_can_post_timer_interrupt(vcpu))