linux-kernel - Re: [PATCH v2 13/31] KVM: x86: hyper-v: Direct TLB flush

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Message-ID: <87lew76eex.fsf@redhat.com>
Date:   Thu, 14 Apr 2022 14:24:38 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 13/31] KVM: x86: hyper-v: Direct TLB flush

Sean Christopherson <seanjc@...gle.com> writes:

> On Thu, Apr 07, 2022, Vitaly Kuznetsov wrote:
>> Handle Direct TLB flush requests from L2 by going through all vCPUs
>
> What is a "Direct TLB flush request" in this context?  I can't tell if "direct"
> refers to the MMU being direct, or if it has some other Hyper-V specific meaning.
>
> Ewww, it looks to be Hyper-V terminology.  Now I see that @direct=true is getting
> L2's ring, not L1's ring.  That's all kinds of evil.  That confusion goes away with
> my suggestion below, but this shortlog and changelog (and the ones for nVMX and
> nSVM enabling) absolutely need to clarify "direct" since it conflicts mightily
> with KVM's "direct" terminology.
>
> In fact, unless I'm missing a patch where "Direct" doesn't mean "From L2", I vote
> to not use the "Direct TLB flush" terminology in any of the shortlogs or changelogs
> and only add a footnote to this first changelog to call out that the TLFS (or
> wherever this terminology came from) calls these types of flushes
> "Direct".

In soon-to-be-sent-out v3 I got rid of 'Direct TLB flush' completely.
Note, in addition to what gets introduced in this series, there are
two other Hyper-V specific places which overload 'direct' already:

- Direct TLB flush for KVM-on-Hyper-V (enable_direct_tlbflush()). I'm
getting rid of it too.

- Direct synthetic timers. 'Direct' in this case means that the timer
signal is delivered via dedicated IRQ 'directly' and not through Vmbus
message. This stays as I can't think of how we can rename it (and if we
should, in the first place).

-- 
Vitaly