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    SubjectRe: [PATCH 3/6] x86/hyper-v: reenlightenment notifications support
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    "Michael Kelley (EOSG)" <Michael.H.Kelley@microsoft.com> writes:

    > On Fri, Dec 08, 2017 at 11:49:57AM +0100, Vitaly Kuznetsov wrote:
    >
    >> -----Original Message-----
    >> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com]
    >> Sent: Friday, December 8, 2017 2:50 AM
    >> To: kvm@vger.kernel.org; x86@kernel.org
    >> Cc: Paolo Bonzini <pbonzini@redhat.com>; Radim Krčmář <rkrcmar@redhat.com>; Thomas
    >> Gleixner <tglx@linutronix.de>; Ingo Molnar <mingo@redhat.com>; H. Peter Anvin
    >> <hpa@zytor.com>; KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
    >> <haiyangz@microsoft.com>; Stephen Hemminger <sthemmin@microsoft.com>; Michael
    >> Kelley (EOSG) <Michael.H.Kelley@microsoft.com>; Andy Lutomirski <luto@kernel.org>;
    >> Mohammed Gamal <mmorsy@redhat.com>; Cathy Avery <cavery@redhat.com>; linux-
    >> kernel@vger.kernel.org; devel@linuxdriverproject.org
    >> Subject: [PATCH 3/6] x86/hyper-v: reenlightenment notifications support
    >>
    >> Hyper-V supports Live Migration notification. This is supposed to be used in conjunction with
    >> TSC emulation: when we are migrated to a host with different TSC frequency for some short
    >> period host emulates our accesses to TSC and sends us an interrupt to notify about the event.
    >> When we're done updating everything we can disable TSC emulation and everything will start
    >> working fast again.
    >>
    >> We didn't need these notifications before as Hyper-V guests are not supposed to use TSC as a
    >> clocksource: in Linux we even mark it as unstable on boot. Guests normally use 'tsc page'
    >> clocksouce and host updates its values on migrations automatically.
    >>
    >> Things change when we want to run nested virtualization: even when we pass through PV
    >> clocksources (kvm-clock or tsc page) to our guests we need to know TSC frequency and when it
    >> changes.
    >>
    >> Hyper-V Top Level Functional Specification (as of v5.0b) wrongly specifies
    >> EAX:BIT(12) of CPUID:0x40000009 as the feature identification bit. The right one to check is
    >> EAX:BIT(13) of CPUID:0x40000003. I was assured that the fix in on the way.
    >>
    >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
    >
    > [snip]
    >
    >> diff --git a/arch/x86/include/asm/irq_vectors.h
    >> b/arch/x86/include/asm/irq_vectors.h
    >> index 67421f649cfa..e71c1120426b 100644
    >> --- a/arch/x86/include/asm/irq_vectors.h
    >> +++ b/arch/x86/include/asm/irq_vectors.h
    >> @@ -103,7 +103,12 @@
    >> #endif
    >>
    >> #define MANAGED_IRQ_SHUTDOWN_VECTOR 0xef
    >> -#define LOCAL_TIMER_VECTOR 0xee
    >> +
    >> +#if IS_ENABLED(CONFIG_HYPERV)
    >> +#define HYPERV_REENLIGHTENMENT_VECTOR 0xee
    >> +#endif
    >> +
    >> +#define LOCAL_TIMER_VECTOR 0xed
    >>
    >> #define NR_VECTORS 256
    >
    > [snip]
    >
    > Since you are pre-allocating a new vector, would you want to update the irq_cpustat_t
    > data structure and your interrupt handler to count the occurrences of these interrupts,
    > and update arch_show_interrupts() to show the count? Then cat /proc/interrupts
    > will show the count. The reenlightenment interrupts will presumably be rare, but so
    > are some of the others that are already counted and displayed, and it seems like
    > consistency should be maintained.

    I could do that.

    The problem with adding this entry to /proc/interrupts, as I see it,
    is that everyone who has CONFIG_HYPERV enabled in their distro will see
    one additional line which 99,9% of the time will be just '0'. I,
    however, see some value in seeing that L1 migration occured.

    I'll add a separate patch to the series and let x86 maintainers decide
    if it's worthy. Thanks,

    --
    Vitaly

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