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

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Date:   Tue, 25 Jun 2019 20:35:12 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Nadav Amit <namit@...are.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        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 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.