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

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Date:	Wed, 30 Jan 2013 08:53:27 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"K. Y. Srinivasan" <kys@...rosoft.com>
Cc:	<olaf@...fle.de>, <bp@...en8.de>, <apw@...onical.com>,
	<x86@...nel.org>, <tglx@...utronix.de>,
	<devel@...uxdriverproject.org>, <gregkh@...uxfoundation.org>,
	<jasowang@...hat.com>, <linux-kernel@...r.kernel.org>,
	<hpa@...or.com>
Subject: Re: [PATCH 2/3] X86: Add a check to catch Xen emulation of
 Hyper-V

>>> On 30.01.13 at 01:51, "K. Y. Srinivasan" <kys@...rosoft.com> 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>
> ---
>  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]);

I'm not convinced that's the right approach - any hypervisor
could do similar emulation, and hence you either want to make
sure you run on Hyper-V (by excluding all others), or you
tolerate using the emulation (which may require syncing up with
the other guest implementations so that shared resources don't
get used by two parties).

I also wonder whether using the Hyper-V emulation (where
useful, there might not be anything right now, but this may
change going forward) when no Xen support is configured
wouldn't be better than not using anything...

Jan

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