linux-kernel - Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB flush

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Message-ID: <20170810185646.GI6524@worktop.programming.kicks-ass.net>
Date:   Thu, 10 Aug 2017 20:56:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     sixiao@...rosoft.com, haiyangz@...rosoft.com,
        Jork.Loeser@...rosoft.com, sthemmin@...rosoft.com,
        torvalds@...ux-foundation.org, luto@...nel.org, hpa@...or.com,
        vkuznets@...hat.com, linux-kernel@...r.kernel.org,
        rostedt@...dmis.org, andy.shevchenko@...il.com, tglx@...utronix.de,
        kys@...rosoft.com, mingo@...nel.org
Cc:     linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/platform] x86/hyper-v: Use hypercall for remote TLB
 flush

On Thu, Aug 10, 2017 at 11:21:49AM -0700, tip-bot for Vitaly Kuznetsov wrote:
> Commit-ID:  2ffd9e33ce4af4e8cfa3e17bf493defe8474e2eb
> Gitweb:     http://git.kernel.org/tip/2ffd9e33ce4af4e8cfa3e17bf493defe8474e2eb
> Author:     Vitaly Kuznetsov <vkuznets@...hat.com>
> AuthorDate: Wed, 2 Aug 2017 18:09:19 +0200
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Thu, 10 Aug 2017 20:16:44 +0200
> 
> x86/hyper-v: Use hypercall for remote TLB flush
> 
> 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.
> 
> 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.

Hold on.. if we don't IPI for TLB invalidation. What serializes our
software page table walkers like fast_gup() ?