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

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Message-ID: <563D1022.9050100@redhat.com>
Date:	Fri, 6 Nov 2015 21:40:02 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	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 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
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