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

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Message-ID: <CALCETrW6sVVQN1DfMvKMpU9u=bpOeS7WZVfwNPYqPqEiysXjFg@mail.gmail.com>
Date:   Mon, 20 Feb 2017 08:55:53 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...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

On Mon, Feb 20, 2017 at 8:51 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>
> On 20/02/2017 17:46, Andy Lutomirski wrote:
>> On Mon, Feb 20, 2017 at 3:05 AM, Paolo Bonzini <pbonzini@...hat.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's still 5% :)

>
>> That being said, vmx_save_host_state() is, um, poorly optimized.
>
> Yeah, but again it doesn't run that often in practical cases.

But it will if someone ever tries to upstream Google's approach of not
emulating anything in the kernel. :)