From: Dexuan Cui <decui@microsoft.com>
To: KY Srinivasan <kys@microsoft.com>,
        "gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "devel@linuxdriverproject.org" <devel@linuxdriverproject.org>,
        "olaf@aepfle.de" <olaf@aepfle.de>,
        "apw@canonical.com" <apw@canonical.com>,
        "vkuznets@redhat.com" <vkuznets@redhat.com>,
        "jasowang@redhat.com" <jasowang@redhat.com>
Subject: RE: [PATCH 5/5] Drivers: hv: vmbus: Implement a clocksource based on
 the TSC page
Thread-Topic: [PATCH 5/5] Drivers: hv: vmbus: Implement a clocksource based on
 the TSC page
Thread-Index: AQHQwpGwHZJNk2cdfEiVMytWk3awsp3kQ4mw
Date: Mon, 20 Jul 2015 12:14:11 +0000
Message-ID: <bd8c7f484f2c475b9020f06166134df6@SIXPR30MB031.064d.mgd.msft.net>
References: <1437363419-3511-1-git-send-email-kys@microsoft.com>
 <1437363447-3554-1-git-send-email-kys@microsoft.com>
 <1437363447-3554-5-git-send-email-kys@microsoft.com>
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> From: devel [mailto:driverdev-devel-bounces...linuxdriverproject.org] On Behalf
> Of K. Y. Srinivasan
> Sent: Monday, July 20, 2015 11:37
> 
> The current Hyper-V clock source is based on the per-partition reference counter
> and this counter is being accessed via s synthetic MSR -
> HV_X64_MSR_TIME_REF_COUNT.
> Hyper-V has a more efficient way of computing the per-partition reference
> counter value that does not involve reading a synthetic MSR. We implement
> a time source based on this mechanism.
> ...
> diff --git a/arch/x86/include/uapi/asm/hyperv.h
> @@ -183,6 +233,22 @@ int hv_init(void)
> 
>  	hv_context.hypercall_page = virtaddr;
> 
> +#ifdef CONFIG_X86_64
> +	if (ms_hyperv.features & HV_X64_MSR_REFERENCE_TSC_AVAILABLE) {
> +		va_tsc = __vmalloc(PAGE_SIZE, GFP_KERNEL, PAGE_KERNEL);
> +		if (!va_tsc)
> +			goto cleanup;
> +		hv_context.tsc_page = va_tsc;
> +
> +		rdmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
> +
> +		tsc_msr.enable = 1;
> +		tsc_msr.guest_physical_address = vmalloc_to_pfn(va_tsc);
> +
> +		wrmsrl(HV_X64_MSR_REFERENCE_TSC, tsc_msr.as_uint64);
> +		clocksource_register_hz(&hyperv_cs_tsc, NSEC_PER_SEC/100);
> +	}
> +#endif

Should we disable the mechanism and vfree() the page in hv_cleanup() for kexec/kdump?

-- Dexuan
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