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    SubjectRe: [PATCH 6/6] Add age out of guest paused flag
    On Tue, Oct 25, 2011 at 03:26:19PM -0400, Eric B Munson wrote:
    > The KVM_GUEST_PAUSED flag will prevent a guest from compaining about a soft
    > lockup but it can mask real soft lockups if the flag isn't cleared when it is
    > no longer relevant. This patch adds a kvm ioctl that the hypervisor will use
    > when it resumes a guest to start a timer for aging out the flag. The time out
    > will be specified by the hypervisor in the ioctl call.
    >
    > Signed-off-by: Eric B Munson <emunson@mgebm.net>
    > ---
    > arch/x86/include/asm/pvclock.h | 2 ++
    > arch/x86/kernel/kvmclock.c | 24 ++++++++++++++++++++++++
    > arch/x86/kvm/x86.c | 9 +++++++++
    > include/linux/kvm.h | 2 ++
    > include/linux/kvm_host.h | 2 ++
    > 5 files changed, 39 insertions(+), 0 deletions(-)
    >
    > diff --git a/arch/x86/include/asm/pvclock.h b/arch/x86/include/asm/pvclock.h
    > index 9312814..e8460b9 100644
    > --- a/arch/x86/include/asm/pvclock.h
    > +++ b/arch/x86/include/asm/pvclock.h
    > @@ -18,6 +18,8 @@ void kvm_set_host_stopped(struct kvm_vcpu *vcpu);
    >
    > bool kvm_check_and_clear_host_stopped(int cpu);
    >
    > +void kvm_clear_guest_paused(struct kvm_vcpu *vcpu, unsigned int length);
    > +
    > /*
    > * Scale a 64-bit delta by scaling and multiplying by a 32-bit fraction,
    > * yielding a 64-bit result.
    > diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
    > index f4fff3d..f3f3935 100644
    > --- a/arch/x86/kernel/kvmclock.c
    > +++ b/arch/x86/kernel/kvmclock.c
    > @@ -23,6 +23,8 @@
    > #include <asm/apic.h>
    > #include <linux/percpu.h>
    > #include <linux/kvm_host.h>
    > +#include <linux/kobject.h>
    > +#include <linux/sysfs.h>
    >
    > #include <asm/x86_init.h>
    > #include <asm/reboot.h>
    > @@ -144,6 +146,28 @@ bool kvm_check_and_clear_host_stopped(int cpu)
    > return ret;
    > }
    >
    > +static void kvm_timer_clear_guest_paused(unsigned long vcpu_addr)
    > +{
    > + struct kvm_vcpu *vcpu = (struct kvm_vcpu *)vcpu_addr;
    > + struct pvclock_vcpu_time_info *src = &vcpu->arch.hv_clock;
    > + src->flags = src->flags & (!PVCLOCK_GUEST_STOPPED);
    > +}
    > +
    > +/*
    > + * Host has resumed the guest, we need to clear the guest paused flag so we
    > + * don't mask any real soft lockups.
    > + */
    > +void kvm_clear_guest_paused(struct kvm_vcpu *vcpu, unsigned int length)
    > +{
    > + if (!timer_pending(&vcpu->flag_timer))
    > + setup_timer(&vcpu->flag_timer,
    > + kvm_timer_clear_guest_paused,
    > + (unsigned long)vcpu);
    > + mod_timer(&vcpu->flag_timer,
    > + jiffies + (length * HZ));
    > +}
    > +EXPORT_SYMBOL_GPL(kvm_clear_guest_paused);

    So this is why the scheme could become awkward. You can't determine a
    value for length that will guarantee that _no_ genuine softlockup is
    omitted.

    Can you think of a way to achieve this?

    Also note, all host code is located in arch/x86/kvm, guest
    code in arch/x86/kernel/kvmclock.c.



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