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

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Date:   Wed, 26 Jun 2019 03:41:37 +0000
From:   Nadav Amit <namit@...are.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Dave Hansen <dave.hansen@...el.com>,
        Peter Zijlstra <peterz@...radead.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 8:35 PM, Andy Lutomirski <luto@...nel.org> wrote:
> 
> On Tue, Jun 25, 2019 at 7:39 PM Nadav Amit <namit@...are.com> wrote:
>>> 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
> 
> In the long run, I think that we're going to want a way for one CPU to
> do a remote flush and then, with appropriate locking, update the
> tlb_gen fields for the remote CPU.  Getting this right may be a bit
> nontrivial.

What do you mean by “do a remote flush”?