Re: [PATCH v7 2/3] KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL

[Tao Xu <tao3.xu@intel.com>]

On 7/17/2019 12:03 AM, Eduardo Habkost wrote:
> On Fri, Jul 12, 2019 at 04:29:06PM +0800, Tao Xu wrote:
>> UMWAIT and TPAUSE instructions use IA32_UMWAIT_CONTROL at MSR index E1H
>> to determines the maximum time in TSC-quanta that the processor can reside
>> in either C0.1 or C0.2.
>>
>> This patch emulates MSR IA32_UMWAIT_CONTROL in guest and differentiate
>> IA32_UMWAIT_CONTROL between host and guest. The variable
>> mwait_control_cached in arch/x86/power/umwait.c caches the MSR value, so
>> this patch uses it to avoid frequently rdmsr of IA32_UMWAIT_CONTROL.
>>
>> Co-developed-by: Jingqi Liu <jingqi.liu@intel.com>
>> Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
>> Signed-off-by: Tao Xu <tao3.xu@intel.com>
>> ---
> [...]
>> +static void atomic_switch_umwait_control_msr(struct vcpu_vmx *vmx)
>> +{
>> +	if (!vmx_has_waitpkg(vmx))
>> +		return;
>> +
>> +	if (vmx->msr_ia32_umwait_control != umwait_control_cached)
>> +		add_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL,
>> +			vmx->msr_ia32_umwait_control,
>> +			umwait_control_cached, false);
> 
> How exactly do we ensure NR_AUTOLOAD_MSRS (8) is still large enough?
> 
> I see 3 existing add_atomic_switch_msr() calls, but the one at
> atomic_switch_perf_msrs() is in a loop.  Are we absolutely sure
> that perf_guest_get_msrs() will never return more than 5 MSRs?
> 

Quote the code of intel_guest_get_msrs:

static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr)
{
[...]
	arr[0].msr = MSR_CORE_PERF_GLOBAL_CTRL;
	arr[0].host = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_guest_mask;
	arr[0].guest = x86_pmu.intel_ctrl & ~cpuc->intel_ctrl_host_mask;
	if (x86_pmu.flags & PMU_FL_PEBS_ALL)
		arr[0].guest &= ~cpuc->pebs_enabled;
	else
		arr[0].guest &= ~(cpuc->pebs_enabled & PEBS_COUNTER_MASK);
	*nr = 1;

	if (x86_pmu.pebs && x86_pmu.pebs_no_isolation) {
[...]
		arr[1].msr = MSR_IA32_PEBS_ENABLE;
		arr[1].host = cpuc->pebs_enabled;
		arr[1].guest = 0;
		*nr = 2;
[...]

There are most 2 msrs now. By default umwait is disabled in KVM. So by 
default there is no MSR_IA32_UMWAIT_CONTROL added into 
add_atomic_switch_msr().

Thanks.
> 
>> +	else
>> +		clear_atomic_switch_msr(vmx, MSR_IA32_UMWAIT_CONTROL);
>> +}
>> +
>>   static void vmx_arm_hv_timer(struct vcpu_vmx *vmx, u32 val)
>>   {
>>   	vmcs_write32(VMX_PREEMPTION_TIMER_VALUE, val);
> [...]
> 
>