Re: RFC: Getting rid of LTR in VMX

[Andy Lutomirski]

On Mon, Feb 20, 2017 at 8:51 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
>
> 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'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. :)