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    SubjectRe: [Patch] Skip cpu_calibrate for kernel running under hypervisors.
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    Hi HPA,

    On Mon, 2010-08-16 at 23:30 -0700, H. Peter Anvin wrote:
    > On 08/16/2010 10:51 PM, Alok Kataria wrote:
    > >>
    > >> I'm somewhat reluctant to take this one, since it assumes all the
    > >> hypervisors act the same. This seems rather inherently wrong. In fact,
    > >> the whole statement is fishy as heck... instead of being dependent on
    > >> AMD and so on,
    > >
    > > The check about being on AMD is something that was already there.
    > >
    >
    > I know it was... and calibrate_cpu() seems to be an AMD-specific
    > function, but that's rather crappy. I'm thinking that perhaps we should
    > make it an x86_init function, then the AMD CPU detection can install it
    > and the vmware hypervisor detection can uninstall it.

    I am planning to add a calibrate_apic function ptr in x86_platform_ops,
    for getting the APIC frequency too directly from the hypervisor. If you
    want I can add this calibrate_cpu function ptr too or is the patch below
    okay for now ?

    Thanks,
    Alok

    >
    > >> this should either be a function pointer or a CPU
    > >> (mis)feature bit.
    > >
    > > In any case, I agree that my previous patch did assume all hypervisors
    > > to be same, which might be wrong. How about using the
    > > X86_FEATURE_TSC_RELIABLE bit for this too ? i.e. Skip cpu_calibrate call
    > > if TSC_RELIABLE bit is set. As of now that bit is set for vmware only.
    > >
    > > Something like the below.
    > >
    > > Signed-off-by: Alok N Kataria <akataria@vmware.com>
    > > Cc: H. Peter Anvin <hpa@zytor.com>
    > >
    > > Index: linux-x86-tree.git/arch/x86/kernel/tsc.c
    > > ===================================================================
    > > --- linux-x86-tree.git.orig/arch/x86/kernel/tsc.c 2010-08-03 12:21:20.000000000 -0700
    > > +++ linux-x86-tree.git/arch/x86/kernel/tsc.c 2010-08-16 21:59:32.000000000 -0700
    > > @@ -927,7 +927,8 @@ void __init tsc_init(void)
    > > }
    > >
    > > if (cpu_has(&boot_cpu_data, X86_FEATURE_CONSTANT_TSC) &&
    > > - (boot_cpu_data.x86_vendor == X86_VENDOR_AMD))
    > > + (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) &&
    > > + !(cpu_has(&boot_cpu_data, X86_FEATURE_TSC_RELIABLE)))
    > > cpu_khz = calibrate_cpu();
    > >
    > > printk("Detected %lu.%03lu MHz processor.\n",
    > >
    >
    > That seems like a much better approach.
    >
    > -hpa
    >



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