Re: [PATCH 2/3] X86: Add a check to catch Xen emulation ofHyper-V

From: Jan Beulich
Date: Wed Jan 30 2013 - 03:53:36 EST


>>> On 30.01.13 at 01:51, "K. Y. Srinivasan" <kys@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> ---
> 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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