Re: [PATCH 3/4] KVM: x86 shared msr infrastructure

Marcelo Tosatti
2009-09-16T21:21:44Z

On Wed, Sep 16, 2009 at 03:45:33PM +0300, Avi Kivity wrote:
> The various syscall-related MSRs are fairly expensive to switch. Currently
> we switch them on every vcpu preemption, which is far too often:
>
> - if we're switching to a kernel thread (idle task, threaded interrupt,
> kernel-mode virtio server (vhost-net), for example) and back, then
> there's no need to switch those MSRs since kernel threasd won't
> be exiting to userspace.
>
> - if we're switching to another guest running an identical OS, most likely
> those MSRs will have the same value, so there's little point in reloading
> them.
>
> - if we're running the same OS on the guest and host, the MSRs will have
> identical values and reloading is unnecessary.
>
> This patch uses the new user return notifiers to implement last-minute
> switching, and checks the msr values to avoid unnecessary reloading.
>
> Signed-off-by: Avi Kivity <avi@redhat.com>
> ---
> arch/x86/include/asm/kvm_host.h | 3 +
> arch/x86/kvm/Kconfig | 1 +
> arch/x86/kvm/x86.c | 79 +++++++++++++++++++++++++++++++++++++++
> 3 files changed, 83 insertions(+), 0 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 45226f0..863bde8 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -799,4 +799,7 @@ int kvm_cpu_has_interrupt(struct kvm_vcpu *vcpu);
> int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu);
> int kvm_cpu_get_interrupt(struct kvm_vcpu *v);
>
> +void kvm_define_shared_msr(unsigned index, u32 msr);
> +void kvm_set_shared_msr(unsigned index, u64 val);
> +
> #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index b84e571..4cd4983 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -28,6 +28,7 @@ config KVM
> select HAVE_KVM_IRQCHIP
> select HAVE_KVM_EVENTFD
> select KVM_APIC_ARCHITECTURE
> + select USER_RETURN_NOTIFIER
> ---help---
> Support hosting fully virtualized guest machines using hardware
> virtualization extensions. You will need a fairly recent
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1d454d9..0e7c40c 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -37,6 +37,7 @@
> #include <linux/iommu.h>
> #include <linux/intel-iommu.h>
> #include <linux/cpufreq.h>
> +#include <linux/user-return-notifier.h>
> #include <trace/events/kvm.h>
> #undef TRACE_INCLUDE_FILE
> #define CREATE_TRACE_POINTS
> @@ -87,6 +88,25 @@ EXPORT_SYMBOL_GPL(kvm_x86_ops);
> int ignore_msrs = 0;
> module_param_named(ignore_msrs, ignore_msrs, bool, S_IRUGO | S_IWUSR);
>
> +#define KVM_NR_SHARED_MSRS 16
> +
> +struct kvm_shared_msrs_global {
> + int nr;
> + struct kvm_shared_msr {
> + u32 msr;
> + u64 value;
> + } msrs[KVM_NR_SHARED_MSRS];
> +};
> +
> +struct kvm_shared_msrs {
> + struct user_return_notifier urn;
> + bool registered;
> + u64 current_value[KVM_NR_SHARED_MSRS];
> +};
> +
> +static struct kvm_shared_msrs_global __read_mostly shared_msrs_global;

Does this assume the MSRs in question are consistent across CPUs?

I guess that is not true with arch_prctl(ARCH_SET_GS/ARCH_GET_GS) ?