linux-kernel - Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush

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Date:   Sat, 20 May 2017 20:23:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        devel@...uxdriverproject.org
Cc:     Stephen Hemminger <sthemmin@...rosoft.com>,
        Jork Loeser <Jork.Loeser@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 08/10] x86/hyper-v: use hypercall for remote TLB flush

On 05/19/2017 07:09 AM, Vitaly Kuznetsov wrote:
> Hyper-V host can suggest us to use hypercall for doing remote TLB flush,
> this is supposed to work faster than IPIs.
> 
> Implementation details: to do HvFlushVirtualAddress{Space,List} hypercalls
> we need to put the input somewhere in memory and we don't really want to
> have memory allocation on each call so we pre-allocate per cpu memory areas
> on boot. These areas are of fixes size, limit them with an arbitrary number
> of 16 (16 gvas are able to specify 16 * 4096 pages).
> 
> pv_ops patching is happening very early so we need to separate
> hyperv_setup_mmu_ops() and hyper_alloc_mmu().
> 
> It is possible and easy to implement local TLB flushing too and there is
> even a hint for that. However, I don't see a room for optimization on the
> host side as both hypercall and native tlb flush will result in vmexit. The
> hint is also not set on modern Hyper-V versions.

Why do local flushes exit?

> +static void hyperv_flush_tlb_others(const struct cpumask *cpus,
> +				    struct mm_struct *mm, unsigned long start,
> +				    unsigned long end)
> +{

What tree will this go through?  I'm about to send a signature change 
for this function for tip:x86/mm.

Also, how would this interact with PCID?  I have PCID patches that I'm 
pretty happy with now, and I'm hoping to support PCID in 4.13.