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

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	"Andy Lutomirski" <luto@kernel.org>
Subject: [PATCH RFC 0/3] x86/kvm: avoid expensive rdmsrs for FS/GS base MSRs
Date: Fri,  2 Mar 2018 11:55:00 +0100	[thread overview]
Message-ID: <20180302105503.24428-1-vkuznets@redhat.com> (raw)

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(-)

-- 
2.14.3