linux-kernel - RE: [PATCH RESEND 1/1] X86: Handle Hyper-V vmbus interrupts as special hypervisor interrupts

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Date:	Thu, 24 Jan 2013 16:15:30 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"KY Srinivasan" <kys@...rosoft.com>
Cc:	"olaf@...fle.de" <olaf@...fle.de>, "bp@...en8.de" <bp@...en8.de>,
	"apw@...onical.com" <apw@...onical.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"jasowang@...hat.com" <jasowang@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH RESEND 1/1] X86: Handle Hyper-V vmbus interrupts as
 special hypervisor interrupts

>>> On 24.01.13 at 17:07, KY Srinivasan <kys@...rosoft.com> wrote:

> 
>> -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@...e.com]
>> Sent: Thursday, January 24, 2013 3:48 AM
>> To: KY Srinivasan
>> 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 RESEND 1/1] X86: Handle Hyper-V vmbus interrupts as
>> special hypervisor interrupts
>> 
>> >>> On 24.01.13 at 02:17, "K. Y. Srinivasan" <kys@...rosoft.com> wrote:
>> > @@ -69,6 +74,11 @@ static void __init ms_hyperv_init_platform(void)
>> >  	       ms_hyperv.features, ms_hyperv.hints);
>> >
>> >  	clocksource_register_hz(&hyperv_cs, NSEC_PER_SEC/100);
>> > +
>> > +	/*
>> > +	 * Setup the IDT for hypervisor callback.
>> > +	 */
>> > +	alloc_intr_gate(HYPERVISOR_CALLBACK_VECTOR,
>> hyperv_callback_vector);
>> 
>> Isn't doing this unconditionally here as problematic as the call to
>> clocksource_register_hz() turned out to be when Xen's Hyper-V
>> shim reacts to the CPUID inquiry above?
> 
> I was not sure what to make this conditional on at run-time. To the extent
> that Xen emulation of Hyper-V is complete, this will be a problem. Does Xen
> return all the "feature bits" of Hyper-V. I could discriminate on a feature 
> that
> Xen does not plan to emulate.

I've no idea what plans there might be, but that's the code
there is currently:

int cpuid_viridian_leaves(unsigned int leaf, unsigned int *eax,
                          unsigned int *ebx, unsigned int *ecx,
                          unsigned int *edx)
{
    struct domain *d = current->domain;

    if ( !is_viridian_domain(d) )
        return 0;

    leaf -= 0x40000000;
    if ( leaf > 6 )
        return 0;

    *eax = *ebx = *ecx = *edx = 0;
    switch ( leaf )
    {
    case 0:
        *eax = 0x40000006; /* Maximum leaf */
        *ebx = 0x7263694d; /* Magic numbers  */
        *ecx = 0x666F736F;
        *edx = 0x76482074;
        break;
    case 1:
        *eax = 0x31237648; /* Version number */
        break;
    case 2:
        /* Hypervisor information, but only if the guest has set its
           own version number. */
        if ( d->arch.hvm_domain.viridian.guest_os_id.raw == 0 )
            break;
        *eax = 1; /* Build number */
        *ebx = (xen_major_version() << 16) | xen_minor_version();
        *ecx = 0; /* SP */
        *edx = 0; /* Service branch and number */
        break;
    case 3:
        /* Which hypervisor MSRs are available to the guest */
        *eax = (CPUID3A_MSR_APIC_ACCESS |
                CPUID3A_MSR_HYPERCALL   |
                CPUID3A_MSR_VP_INDEX);
        break;
    case 4:
        /* Recommended hypercall usage. */
        if ( (d->arch.hvm_domain.viridian.guest_os_id.raw == 0) ||
             (d->arch.hvm_domain.viridian.guest_os_id.fields.os < 4) )
            break;
        *eax = (CPUID4A_MSR_BASED_APIC |
                CPUID4A_RELAX_TIMER_INT);
        *ebx = 2047; /* long spin count */
        break;
    }

    return 1;
}

Question is - considering you stated that this is supported
starting in Win8, doesn't Hyper-V itself announce that
capability in some explicit way?

Jan

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