Re: RFC: Getting rid of LTR in VMX

From: Paolo Bonzini
Date: Mon Feb 20 2017 - 11:51:38 EST

On 20/02/2017 17:46, Andy Lutomirski wrote:
> On Mon, Feb 20, 2017 at 3:05 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>> On 18/02/2017 04:29, Andy Lutomirski wrote:
>>> There's no code here because the patch is trivial, but I want to run
>>> the idea by you all first to see if there are any issues.
>>> VMX is silly and forces the TSS limit to the minimum on VM exits. KVM
>>> wastes lots of cycles bumping it back up to accomodate the io bitmap.
>> Actually looked at the code now...
>> reload_tss is only invoked for userspace exits, so it is a nice-to-have
>> but it wouldn't show on most workloads. Still it does save 150-200
>> clock cycles to remove it (I just commented out reload_tss() from
>> __vmx_load_host_state to test).
> That's for anything involving userspace or preemption, right?

Yes. But 150-200 clock cycles are nothing compared to the cache misses
you get from preemption, so I'd ignore that. Saving 300 clock cycles on
userspace exits from TR+GSBASE would be about 5% on my Haswell.

> That being said, vmx_save_host_state() is, um, poorly optimized.

Yeah, but again it doesn't run that often in practical cases.