From: Paolo Bonzini <pbonzini@redhat.com>
To: Andy Lutomirski <luto@amacapital.net>
Cc: kvm list <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
X86 ML <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
Borislav Petkov <bpetkov@suse.de>
Subject: Re: RFC: Getting rid of LTR in VMX
Date: Mon, 20 Feb 2017 17:51:27 +0100 [thread overview]
Message-ID: <a36e6fd4-f5fb-174c-0df6-00a01c353cd5@redhat.com> (raw)
In-Reply-To: <CALCETrUU5Co-gKzsu9ZhVGOBSikLzxm7_6ZoAYVYU5E6SSnV1g@mail.gmail.com>
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
next prev parent reply other threads:[~2017-02-20 16:51 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-02-18 3:29 RFC: Getting rid of LTR in VMX Andy Lutomirski
2017-02-18 15:19 ` Paolo Bonzini
2017-02-20 11:05 ` Paolo Bonzini
2017-02-20 16:46 ` Andy Lutomirski
2017-02-20 16:51 ` Paolo Bonzini [this message]
2017-02-20 16:55 ` Andy Lutomirski
2017-02-20 22:02 ` Paolo Bonzini
2017-02-20 22:23 ` hpa
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