linux-kernel - Re: [PATCH] x86/hyper-v: use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS

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Date:   Tue, 19 Jun 2018 14:05:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
cc:     x86@...nel.org, devel@...uxdriverproject.org,
        linux-kernel@...r.kernel.org,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Tianyu Lan <Tianyu.Lan@...rosoft.com>
Subject: Re: [PATCH] x86/hyper-v: use cheaper HVCALL_FLUSH_VIRTUAL_ADDRESS_{LIST,SPACE}
 hypercalls when possible

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.

Thanks,

	tglx