linux-kernel - Re: RFC: Getting rid of LTR in VMX

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Message-ID: <890292613.23897064.1487628173163.JavaMail.zimbra@redhat.com>
Date:   Mon, 20 Feb 2017 17:02:53 -0500 (EST)
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     kvm list <kvm@...r.kernel.org>, linux-kernel@...r.kernel.org,
        X86 ML <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Borislav Petkov <bpetkov@...e.de>
Subject: Re: RFC: Getting rid of LTR in VMX


> > 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's still 5% :)

Yes, 5% on userspace exits is good (though they're rare).

OTOH, 300 clock cycles on preemption are nothing to write home about.

Paolo