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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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