Re: [PATCH] x86/hyper-v: use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST, SPACE} hypercalls when possible

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Thomas Gleixner <tglx@linutronix.de> writes:

> On Tue, 19 Jun 2018, Vitaly Kuznetsov wrote:
>> Thomas Gleixner <tglx@linutronix.de> writes:
>> 
>> > On Fri, 15 Jun 2018, Vitaly Kuznetsov wrote:
>> >>   * Fills in gva_list starting from offset. Returns the number of items added.
>> >> @@ -93,10 +95,19 @@ static void hyperv_flush_tlb_others(const struct cpumask *cpus,
>> >>  	if (cpumask_equal(cpus, cpu_present_mask)) {
>> >>  		flush->flags |= HV_FLUSH_ALL_PROCESSORS;
>> >>  	} else {
>> >> +		/*
>> >> +		 * It is highly likely that VP ids are in ascending order
>> >> +		 * matching Linux CPU ids; Check VP index for the highest CPU
>> >> +		 * in the supplied set to see if EX hypercall is required.
>> >> +		 * This is just a best guess but should work most of the time.
>> >
>> > TLB flushing based on 'best guess' and 'should work most of the time' is
>> > not a brilliant approach.
>> >
>> 
>> Oh no no no, that's not what I meant :-)
>> 
>> We have the following problem: from the supplied CPU set we need to
>> figure out if we can get away with HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,
>> SPACE} hypercalls which are cheaper or if we need to use more expensing
>> HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST, SPACE}_EX ones. The dividing line is
>> the highest VP_INDEX of the supplied CPU set: in case it is < 64 cheaper
>> hypercalls are OK. Now how do we check that? In the patch I have the
>> following approach:
>> 1) Check VP number for the highest CPU in the supplied set. In case it
>> is > 64 we for sure need more expensive hypercalls. This is the "guess"
>> which works most of the time because Linux CPU ids usually match
>> VP_INDEXes.
>> 
>> 2) In case the answer to the previous question was negative we start
>> preparing input for the cheaper hypercall. However, if while walking the
>> CPU set we meet a CPU with VP_INDEX higher than 64 we'll discard the
>> prepared input and switch to the more expensive hypercall.
>> 
>> Said that the 'guess' here is just an optimization to avoid walking the
>> whole CPU set when we find the required answer quickly by looking at the
>> highest bit. This will help big systems with hundreds of CPUs.
>
> Care to fix the comment to avoid the offending words?
>

Sure, will re-word in v2.

-- 
  Vitaly