linux-kernel - Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on Hyper-V

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Date:   Thu, 15 Mar 2018 20:43:47 +0100
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        x86@...nel.org, "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 7/7] x86/kvm: use Enlightened VMCS when running on
 Hyper-V

2018-03-15 20:28+0100, Thomas Gleixner:
> On Thu, 15 Mar 2018, Radim Krčmář wrote:
> > 2018-03-15 16:19+0100, Vitaly Kuznetsov:
> > > This works. But hell, this is a crude hack :-) Not sure if there's a
> > > cleaner way to find what needs to be patched without something like jump
> > > label table ...
> > 
> > Yeah, I can see us accidently patching parts of other instructions. :)
> > 
> > The target instruction address can be made into a C-accessible symbol
> > with the same trick that vmx_return uses -- add a .global containing the
> > address of a label (not sure if a more direct approach would work).
> > 
> > The evil in me likes it.  (The good is too lazy to add a decent patching
> > infrastructure for just one user.)
> 
> Can we just use jump labels please? There is agreement that 4.17 will have
> a dependency on a jump label capable compiler for x86.

Luckily, it turned out that the path is very cold and should use the
simple test-and-jump.