linux-kernel - Re: Speeding up VMX with GDT fixmap trickery?

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Date:   Fri, 9 Jun 2017 08:45:12 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Garnier <thgarnie@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: Speeding up VMX with GDT fixmap trickery?

On Fri, Jun 9, 2017 at 12:14 AM, Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>
> On 09/06/2017 03:13, Andy Lutomirski wrote:
>> Hi all-
>>
>> As promised when Thomas did his GDT fixmap work, here is a draft patch
>> to speed up KVM by extending it.
>>
>> The downside of this patch is that it makes the fixmap significantly
>> larger on 64-bit systems if NR_CPUS is large (it adds 15 more pages
>> per CPU).  I don't know if we care at all.  It also bloats the kernel
>> image by 4k and wastes 4k of RAM for the entire time the system is
>> booted.  We could avoid the latter bit (sort of) by not mapping the
>> extra fixmap pages at all and handling the resulting faults somehow.
>> That would scare me -- now we have IRET generating #PF when running
>> malicious , and that way lies utter madness.
>>
>> The upside is that we don't need to do LGDT after a vmexit on VMX.
>> LGDT is slooooooooooow.  But no, I haven't benchmarked this yet.
>>
>> What do you all think?
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/kvm&id=e249a09787d6956b52d8260b2326d8f12f768799
>
> Not sure I understand this completely, but:
>
>  /* Get the fixmap index for a specific processor */
>  static inline unsigned int get_cpu_gdt_ro_index(int cpu)
>  {
> -       return FIX_GDT_REMAP_BEGIN + cpu;
> +       return FIX_GDT_REMAP_END - cpu * PAGES_PER_GDT;
>  }
>
> isn't this off by one.  I think it should be
>
>         FIX_GDT_REMAP_END + 1 - cpu * PAGES_PER_GDT
>
> or just FIX_GDT_REMAP_BEGIN + cpu * PAGES_PER_GDT?  That is for example:
>
>         FIX_GDT_REMAP_BEGIN = 100
>         get_cpu_gdt_ro_index(0) = 100
>         get_cpu_gdt_ro_index(1) = 116
>         get_cpu_gdt_ro_index(2) = 132
>         get_cpu_gdt_ro_index(3) = 148
>         FIX_GDT_REMAP_END = 163

The issue here is that the fixmap is upside down: lower indices are
*higher* addresses, which means that, if we have a multi-page GDT, we
need get_cpu_gdt_ro_index() to return an index of the lowest page in
each GDT.  The simplest way seems to be to put them in ascending
order.

With the range of indices being 100 .. 163 (with 4 CPUs), we'd want
the GDTs to be at:

163..148
147..132
131..116
115..100

so FIX_GDT_REMAP_END - cpu * PAGES_PER_GDT is correct, I think.  Or am
I still off by one?

--Andy