linux-kernel - Re: [PATCH] x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only

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Date:	Wed, 24 Sep 2014 21:39:41 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	chris@...chsys.com, Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH] x86: kvm: use alternatives for VMCALL vs. VMMCALL if
 kernel text is read-only

On Mon, 22 Sep 2014, Paolo Bonzini wrote:

> On x86_64, kernel text mappings are mapped read-only with CONFIG_DEBUG_RODATA.
> In that case, KVM will fail to patch VMCALL instructions to VMMCALL
> as required on AMD processors.
>
> The failure mode is currently a divide-by-zero exception, which obviously
> is a KVM bug that has to be fixed.  However, picking the right instruction
> between VMCALL and VMMCALL will be faster and will help if you cannot upgrade
> the hypervisor.
>
> -/* This instruction is vmcall.  On non-VT architectures, it will generate a
> - * trap that we will then rewrite to the appropriate instruction.
> +#ifdef CONFIG_DEBUG_RODATA
> +#define KVM_HYPERCALL \
> +        ALTERNATIVE(".byte 0x0f,0x01,0xc1", ".byte 0x0f,0x01,0xd9", X86_FEATURE_VMMCALL)

If we can do it via a feature bit and alternatives, then why do you
want to patch it manually if CONFIG_DEBUG_RODATA=n?

Just because more #ifdeffery makes the code more readable?

Thanks,

	tglx
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