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

From: Paolo Bonzini
Date: Fri Nov 06 2015 - 15:40:15 EST

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
>>> 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@xxxxxxxxxx>
>> 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!

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