linux-kernel - Re: [PATCH v2 00/12] KVM: x86: add support for VMX TSC scaling

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Message-ID: <20151109000623.GB27132@hzzhang-OptiPlex-9020.sh.intel.com>
Date:	Mon, 9 Nov 2015 08:06:23 +0800
From:	haozhong.zhang@...el.com
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	<kvm@...r.kernel.org>, Gleb Natapov <gleb@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
	Joerg Roedel <joro@...tes.org>,
	Wanpeng Li <wanpeng.li@...mail.com>,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	<mdontu@...defender.com>, Kai Huang <kai.huang@...ux.intel.com>,
	"Andy Lutomirski" <luto@...nel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 00/12] KVM: x86: add support for VMX TSC scaling

On 11/06/15 21:40, Paolo Bonzini wrote:
> 
> 
> On 06/11/2015 13:42, Haozhong Zhang wrote:
> > On 11/06/15 11:49, Paolo Bonzini wrote:
> >>
> >>
> >> On 20/10/2015 09:39, Haozhong Zhang wrote:
> >>> This patchset adds support for VMX TSC scaling feature which is
> >>> available on Intel Skylake CPU. The specification of VMX TSC scaling
> >>> can be found at
> >>> http://www.intel.com/content/www/us/en/processors/timestamp-counter-scaling-virtualization-white-paper.html
> >>>
> >>> VMX TSC scaling allows guest TSC which is read by guest rdtsc(p)
> >>> instructions increases in a rate that is customized by the hypervisor
> >>> and can be different than the host TSC rate. Basically, VMX TSC
> >>> scaling adds a 64-bit field called TSC multiplier in VMCS so that, if
> >>> VMX TSC scaling is enabled, TSC read by guest rdtsc(p) instructions
> >>> will be calculated by the following formula:
> >>>
> >>>   guest EDX:EAX = (Host TSC * TSC multiplier) >> 48 + VMX TSC Offset
> >>>
> >>> where, Host TSC = Host MSR_IA32_TSC + Host MSR_IA32_TSC_ADJUST.
> >>>
> >>> This patchset, when cooperating with another QEMU patchset (sent in
> >>> another email "target-i386: save/restore vcpu's TSC rate during
> >>> migration"), allows guest programs observe a consistent TSC rate even
> >>> though they are migrated among machines with different host TSC rates.
> >>>
> >>> VMX TSC scaling shares some common logics with SVM TSC ratio which
> >>> is already supported by KVM. Patch 1 ~ 8 move those common logics from
> >>> SVM code to the common code. Upon them, patch 9 ~ 12 add VMX-specific
> >>> support for VMX TSC scaling.
> >>>
> >>> Changes in v2:
> >>>  * Remove the duplicated variable 'kvm_tsc_scaling_ratio_rsvd'.
> >>>  * Remove an unnecessary error check in original patch 2.
> >>>  * Do 64-bit arithmetic by functions recommended by Paolo.
> >>>  * Make kvm_set_tsc_khz() returns an error number so that ioctl
> >>>    KVM_SET_TSC_KHZ does not return 0 if errors happen.
> >>>
> >>> Reviewed-by: Eric Northup <digitaleric@...gle.com>
> >>
> >> Thanks for the patches.  There are a couple changes that I can do myself:
> >>
> >> 1) kvm_default_tsc_scaling_ratio can be initialized in
> >> kvm_arch_hardware_setup, since it's just 1ULL <<
> >> kvm_tsc_scaling_ratio_frac_bits
> >>
> > Agree
> > 
> >> 2) things that you are adding to include/linux/kvm_host.h should instead
> >> go in arch/x86/include/linux/kvm_host.h
> >>
> > Agree, because they are x86 specific.
> > 
> >> That's it.  I'll commit it as soon as I test on AMD (today hopefully).
> 
> It tested fine.  I'll give it a shot with the 32-bit mul_u64_u64_shr on
> Monday as well, but I don't expect any issue.
> 
> Thanks, the patches are neat!
> 
> Paolo

Thank you for the test!

Haozhong

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