linux-kernel - Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

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Date:	Wed, 17 Apr 2013 10:06:16 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	x86@...nel.org, gregkh@...uxfoundation.org,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	olaf@...fle.de, apw@...onical.com, jasowang@...hat.com,
	tglx@...utronix.de, hpa@...or.com, JBeulich@...e.com, bp@...en8.de
Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of Hyper-V

On Sun, Feb 03, 2013 at 05:22:38PM -0800, K. Y. Srinivasan wrote:
> Xen emulates Hyper-V to host enlightened Windows. Looks like this
> emulation may be turned on by default even for Linux guests. Check and
> fail Hyper-V detection if we are on Xen.
> 
> Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>

I'm very curious what's the specific bug that is fixed here?
I went over the original discussion in
https://patchwork.kernel.org/patch/2064331/
and that's still not clear to me. Is there a configuration
that is broken without this patch but starts working with
this patch?

It seems that one might want to use hyper-v emulation e.g. to test
hyper-v code without using windows, so the functionality
that this patch disables is not completely useless,
so there should be a good reason for disabling it.

Could you enlighten me please? :)

> ---
>  arch/x86/kernel/cpu/mshyperv.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mshyperv.c b/arch/x86/kernel/cpu/mshyperv.c
> index 646d192..4dab317 100644
> --- a/arch/x86/kernel/cpu/mshyperv.c
> +++ b/arch/x86/kernel/cpu/mshyperv.c
> @@ -30,6 +30,13 @@ static bool __init ms_hyperv_platform(void)
>  	if (!boot_cpu_has(X86_FEATURE_HYPERVISOR))
>  		return false;
>  
> +	/*
> +	 * Xen emulates Hyper-V to support enlightened Windows.
> +	 * Check to see first if we are on a Xen Hypervisor.
> +	 */
> +	if (xen_cpuid_base())
> +		return false;
> +
>  	cpuid(HYPERV_CPUID_VENDOR_AND_MAX_FUNCTIONS,
>  	      &eax, &hyp_signature[0], &hyp_signature[1], &hyp_signature[2]);
>  
> -- 
> 1.7.4.1
> 
> --
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