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

[Marcelo Tosatti <mtosatti@redhat.com>]

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) ?