Re: [patch V2 17/17] x86/kvm: Use generic exit to guest work function

Sean Christopherson
2019-10-23T14:48:51Z

On Wed, Oct 23, 2019 at 02:27:22PM +0200, Thomas Gleixner wrote:
> Use the generic infrastructure to check for and handle pending work before
> entering into guest mode.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> arch/x86/kvm/Kconfig | 1 +
> arch/x86/kvm/x86.c | 17 +++++------------
> 2 files changed, 6 insertions(+), 12 deletions(-)
>
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -42,6 +42,7 @@ config KVM
> select HAVE_KVM_MSI
> select HAVE_KVM_CPU_RELAX_INTERCEPT
> select HAVE_KVM_NO_POLL
> + select KVM_EXIT_TO_GUEST_WORK
> select KVM_GENERIC_DIRTYLOG_READ_PROTECT
> select KVM_VFIO
> select SRCU
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -52,6 +52,7 @@
> #include <linux/irqbypass.h>
> #include <linux/sched/stat.h>
> #include <linux/sched/isolation.h>
> +#include <linux/entry-common.h>
> #include <linux/mem_encrypt.h>
>
> #include <trace/events/kvm.h>
> @@ -8115,8 +8116,8 @@ static int vcpu_enter_guest(struct kvm_v
> if (kvm_lapic_enabled(vcpu) && vcpu->arch.apicv_active)
> kvm_x86_ops->sync_pir_to_irr(vcpu);
>
> - if (vcpu->mode == EXITING_GUEST_MODE || kvm_request_pending(vcpu)
> - || need_resched() || signal_pending(current)) {
> + if (vcpu->mode == EXITING_GUEST_MODE || kvm_request_pending(vcpu) ||
> + exit_to_guestmode_work_pending()) {

The terms EXIT_TO_GUEST and exit_to_guestmode are very confusing, as
they're inverted from the usual virt terminology of VM-Enter (enter guest)
and VM-Exit (exit guest). The conflict is most obvious here, with the
above "vcpu->mode == EXITING_GUEST_MODE", which is checking to see if the
vCPU is being forced to exit *from* guest mode because was kicked by some
other part of KVM.

Maybe XFER_TO_GUEST? I.e. avoid entry/exit entirely, so that neither the
entry code or KVM ends up with a confusing name.

> vcpu->mode = OUTSIDE_GUEST_MODE;
> smp_wmb();
> local_irq_enable();
> @@ -8309,17 +8310,9 @@ static int vcpu_run(struct kvm_vcpu *vcp
>
> kvm_check_async_pf_completion(vcpu);
>
> - if (signal_pending(current)) {
> - r = -EINTR;
> - vcpu->run->exit_reason = KVM_EXIT_INTR;
> - ++vcpu->stat.signal_exits;
> + r = exit_to_guestmode(kvm, vcpu);

Ditto here. If the run loop is stripped down to the core functionality,
it effectively looks like:

for (;;) {
r = vcpu_enter_guest(vcpu);
if (r <= 0)
break;

...

r = exit_to_guestmode(kvm, vcpu);
if (r)
break;
}

Appending _handle_work to the function would also be helpful so that it's
somewhat clear the function isn't related to the core vcpu_enter_guest()
functionality, e.g.:

for (;;) {
r = vcpu_enter_guest(vcpu);
if (r <= 0)
break;

...

r = xfer_to_guestmode_handle_work(kvm, vcpu);
if (r)
break;
}

> + if (r)
> break;
> - }
> - if (need_resched()) {
> - srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
> - cond_resched();
> - vcpu->srcu_idx = srcu_read_lock(&kvm->srcu);
> - }
> }
>
> srcu_read_unlock(&kvm->srcu, vcpu->srcu_idx);
>
>