linux-kernel - Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by PUD

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Date:   Tue, 20 Aug 2019 14:14:10 +0000
From:   Song Liu <songliubraving@...com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Joerg Roedel <jroedel@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Peter Zijlstra" <peterz@...radead.org>
Subject: Re: [PATCH] x86/mm/pti: in pti_clone_pgtable() don't increase addr by
 PUD_SIZE



> On Aug 20, 2019, at 6:57 AM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 8/20/19 12:51 AM, Song Liu wrote:
>> In our x86_64 kernel, pti_clone_pgtable() fails to clone 7 PMDs because
>> of this issuse, including PMD for the irq entry table. For a memcache
>> like workload, this introduces about 4.5x more iTLB-load and about 2.5x
>> more iTLB-load-misses on a Skylake CPU.
> 
> I was surprised that this manifests as a performance issue.  Usually
> messing up PTI page table manipulation means you get to experience the
> jobs of debugging triple faults.  But, it makes sense if its this line:
> 
>        /*
>         * Note that this will undo _some_ of the work that
>         * pti_set_kernel_image_nonglobal() did to clear the
>         * global bit.
>         */
>        pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE);
> 
> which is restoring the Global bit.
> 
> *But*, that shouldn't get hit on a Skylake CPU since those have PCIDs
> and shouldn't have a global kernel image.  Could you confirm whether
> PCIDs are supported on this CPU?

Yes, pcid is listed in /proc/cpuinfo. 

Thanks,
Song