linux-kernel - Re: Feedback on 4.9 performance after PTI fixes

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Date:   Mon, 8 Jan 2018 19:26:22 +0100
From:   Willy Tarreau <w@....eu>
To:     Yves-Alexis Perez <corsac@...ian.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: Feedback on 4.9 performance after PTI fixes

Hi Yves-Alexis,

On Mon, Jan 08, 2018 at 06:07:54PM +0100, Yves-Alexis Perez wrote:
> On Sun, 2018-01-07 at 11:18 +0100, Willy Tarreau wrote:
> >   - the highest performance impact on VMs comes from having PTI on the
> >     guest kernel (-45%). At this point it makes no difference whether
> >     the host kernel has it or not.
> 
> Hi Willy,
> 
> out of curiosity, is the pcid/invpcid flags exposed to and used by your guest
> CPU? It might very well that the PCID optimisations are not used by the guests
> here, and it might be worth either checking on bare metal or with the PCID
> optimisations enabled.

You're totally right, I discovered during my later developments that
indeed PCID is not exposed there. So we take the hit of a full TLB
flush twice per syscall.

Willy