linux-kernel - Re: [PATCH 12/13] KVM: x86: add KVM_MEM_X86

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Date:	Wed, 06 May 2015 21:43:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
CC:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org, bsd@...hat.com,
	guangrong.xiao@...ux.intel.com,
	Yang Zhang <yang.z.zhang@...el.com>, wanpeng.li@...ux.intel.com
Subject: Re: [PATCH 12/13] KVM: x86: add KVM_MEM_X86_SMRAM memory slot flag

On 06/05/2015 18:24, Radim Krčmář wrote:
> The feature you wanted exposed a flaw in the code, so an extension was
> needed.  Copying code is the last resort after all options of
> abstracting were exhausted ... I might be forcing common paths when
> writing it twice requires less brain power, but 200 lines of
> structurally identical code seem far from it.

Note that it didn't really expose a flaw in the code, just a limitation.
 There are cases even on x86 where you have no vcpu, for example filling
in the EPT identity page tables or VMX TSS.

> Reworking stable code is simpler, as we can just cover all features
> needed now and omit the hard thinking about future extensions.
> (For me, stable code is the first candidate for generalization ...
>  and I wouldn't copy it, even though it's mostly fine in practice.)

Stable simple code is also important to keep simple though.  Sometimes
code duplication is preferrable to obfuscation.

I agree that copying 200 lines of code because of one function three
levels down the call chain isn't nice.  However, it doesn't seem
particularly easy to avoid the duplication even with C++ templates.  C
is worse.  OCaml or Haskell would be nicer. :)

Paolo
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