[PATCH RFC 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs

[Vitaly Kuznetsov]

Some time ago Paolo suggested to take a look at probably unneeded expensive
rdmsrs for FS/GS base MSR in vmx_save_host_state(). This is called on every
vcpu run when we need to handle vmexit in userspace.

I have to admit I got a bit lost in our kernel FS/GS magic. I managed to 
convince myself that in the well defined context (ioctl from userspace)
we can always get the required values from in-kernel variables and avoid
rdmsrs. But I may have missed something really important, thus RFC.

My debug shows we're shaving off 240 cpu cycles (E5-2603 v3).

In case these patches turn out to be worthwile AMD SVM can probably be
optimized the ame way.

Vitaly Kuznetsov (3):
  x86/kvm/vmx: read MSR_FS_BASE from current->thread
  x86/kvm/vmx: read MSR_KERNEL_GS_BASE from current->thread
  x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE

 arch/x86/kernel/cpu/common.c | 1 +
 arch/x86/kvm/vmx.c           | 7 ++++---
 2 files changed, 5 insertions(+), 3 deletions(-)

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2.14.3