linux-kernel - Re: [PATCH 6/9] KVM: x86: Provide paravirtualized flush_tlb

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Date:   Wed, 26 Jun 2019 02:39:45 +0000
From:   Nadav Amit <namit@...are.com>
To:     Dave Hansen <dave.hansen@...el.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH 6/9] KVM: x86: Provide paravirtualized flush_tlb_multi()

> On Jun 25, 2019, at 2:40 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 6/12/19 11:48 PM, Nadav Amit wrote:
>> Support the new interface of flush_tlb_multi, which also flushes the
>> local CPU's TLB, instead of flush_tlb_others that does not. This
>> interface is more performant since it parallelize remote and local TLB
>> flushes.
>> 
>> The actual implementation of flush_tlb_multi() is almost identical to
>> that of flush_tlb_others().
> 
> This confused me a bit.  I thought we didn't support paravirtualized
> flush_tlb_multi() from reading earlier in the series.
> 
> But, it seems like that might be Xen-only and doesn't apply to KVM and
> paravirtualized KVM has no problem supporting flush_tlb_multi().  Is
> that right?  It might be good to include some of that background in the
> changelog to set the context.

I’ll try to improve the change-logs a bit. There is no inherent reason for
PV TLB-flushers not to implement their own flush_tlb_multi(). It is left
for future work, and here are some reasons:

1. Hyper-V/Xen TLB-flushing code is not very simple
2. I don’t have a proper setup
3. I am lazy