linux-kernel - Re: [PATCH v2 0/7] KVM TSS cleanups and speedups

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Date:   Wed, 22 Feb 2017 16:13:46 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>
Cc:     kvm list <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Borislav Petkov <bpetkov@...e.de>,
        Thomas Garnier <thgarnie@...gle.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2 0/7] KVM TSS cleanups and speedups

On 21/02/2017 20:14, Andy Lutomirski wrote:
> The first four patches here are intended to be straightforward
> cleanups and to make a better base for Thomas' GDT series.  They may
> be a slight speedup, too, because they remove an STR instruction
> from the VMX entry path.
> 
> The last two patches are a reasonably large speedup but need careful
> review.
> 
> FWIW, I can see lots of additional easy-ish speedups here.  For example:
> 
>  - The GDT reload on VM exit isn't really needed at all.  Instead let's
>    just change the kernel limit to 0xFFFF.  Doing that naively would
>    waste memory, but doing it carefully on top of Thomas' series would
>    be straightforward and almost free.
> 
>  - RDMSR from MSR_GS_BASE is totally pointless.
> 
>  - Once I or someone finishes the FSGSBASE series, we get a big speedup
>    there.
> 
>  - The LDT reload code should be split up and optimized better, I think.
> 
> Changes from v1:
>  - Fix some changelog typos.
>  - Fix the bug that Paolo found.
>  - Rename the helpers to make their usage more obvious.
>  - Move clearing __tss_limit_invalid into force_reload_TR() as a tiny
>    optimization.
>  - Add a test case.  It doesn't test all the machinations, but at least
>    it checks basic functionality.
> 
> Andy Lutomirski (7):
>   x86/asm: Define the kernel TSS limit in a macro
>   x86/kvm/vmx: Don't fetch the TSS base from the GDT
>   x86/kvm/vmx: Get rid of segment_base() on 64-bit kernels
>   x86/kvm/vmx: Simplify segment_base()
>   x86/asm/64: Drop __cacheline_aligned from struct x86_hw_tss
>   x86/kvm/vmx: Defer TR reload after VM exit
>   selftests/x86: Add a basic selftest for ioperm
> 
>  arch/x86/include/asm/desc.h          |  62 +++++++++++--
>  arch/x86/include/asm/processor.h     |  12 ++-
>  arch/x86/kernel/ioport.c             |  11 +++
>  arch/x86/kernel/process.c            |  10 ++
>  arch/x86/kvm/vmx.c                   |  63 ++++++-------
>  tools/testing/selftests/x86/Makefile |   2 +-
>  tools/testing/selftests/x86/ioperm.c | 171 +++++++++++++++++++++++++++++++++++
>  7 files changed, 284 insertions(+), 47 deletions(-)
>  create mode 100644 tools/testing/selftests/x86/ioperm.c
> 

I pushed and tagged before seeing this v2. :(  The differences seem to
be x86-only, so I suppose Ingo can handle them if you resubmit.

Paolo