linux-kernel - Re: [PATCH 2/3] x86: Get TSC frequency from VMware hypervisor.

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Message-ID: <48FD35CC.10904@zytor.com>
Date:	Mon, 20 Oct 2008 18:52:12 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	akataria@...are.com
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dan Hecht <dhecht@...are.com>
Subject: Re: [PATCH 2/3] x86: Get TSC frequency from VMware hypervisor.

Alok Kataria wrote:
> x86: Get TSC frequency from VMware hypervisor.
> 
> From: Alok N Kataria <akataria@...are.com>
> 
> This patch adds functions to detect if we are running under VMware.
> The current way to check if we are on VMware is following,
> #  check if "hypervisor present bit" is set, if so read the 0x40000000
>    cpuid leaf and check for "VMwareVMware" signature.
> #  if the above fails, check the DMI vendors name for "VMware" string
>    if we find one we query the VMware backdoor port to check if we are
>    under VMware.
> 
> The DMI + Backdoor check is needed for older VMware products, which
> don't implement the hypervisor signature cpuid leaf.
> Also note that since we are checking for the DMI signature the backdoor
> port would never be accessed on native hardware.
> 
> This patch also adds a hypervisor_get_tsc_freq function, instead of
> calibrating the frequency which can be error prone in virtualized
> environment, we ask the hypervisor for it. We get the frequency from
> the hypervisor by accessing the backdoor port if we are running on VMware.
> Other hypervisors too can add code to get frequency on their platform
> to this routine.
> 

I would like to see, instead of calling vmware_platform() directly in 
places like tsc.c, a hypervisor field in the CPU structure that is set 
with the rest of the CPU identification stuff.  That way we avoid ending 
up with garbage like:

if (vmware_platform() || xen_platform() || kvm_platform() ...)

	-hpa
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