Messages in this thread | | | Subject | Re: RFC: Getting rid of LTR in VMX | From | Paolo Bonzini <> | Date | Mon, 20 Feb 2017 17:51:27 +0100 |
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On 20/02/2017 17:46, Andy Lutomirski wrote: > On Mon, Feb 20, 2017 at 3:05 AM, Paolo Bonzini <pbonzini@redhat.com> 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.
Paolo
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