linux-kernel - Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes

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Message-ID: <d68bef6d-d71c-7881-87e8-133f657e495a@intel.com>
Date:   Tue, 27 Aug 2019 11:28:53 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Nadav Amit <namit@...are.com>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [RFC PATCH v2 2/3] x86/mm/tlb: Defer PTI flushes

On 8/23/19 3:52 PM, Nadav Amit wrote:
> INVPCID is considerably slower than INVLPG of a single PTE. Using it to
> flush the user page-tables when PTI is enabled therefore introduces
> significant overhead.

I'm not sure this is worth all the churn, especially in the entry code.
 For large flushes (> tlb_single_page_flush_ceiling), we don't do
INVPCIDs in the first place.

I'd really want to understand what the heck is going on that makes
INVPCID so slow, first.