linux-kernel - Re: [PATCH v2 00/13] perf: KVM: Fix, optimize, and clean up callbacks

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Message-ID: <0eb2d4b1-23a4-c318-9f91-8dce78c6c8ad@redhat.com>
Date:   Mon, 20 Sep 2021 20:22:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Guo Ren <guoren@...nel.org>, Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        kvm@...r.kernel.org, xen-devel@...ts.xenproject.org,
        Artem Kashkanov <artem.kashkanov@...el.com>,
        Like Xu <like.xu.linux@...il.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>
Subject: Re: [PATCH v2 00/13] perf: KVM: Fix, optimize, and clean up callbacks

On 20/09/21 15:40, Marc Zyngier wrote:
>>> At least not before we
>>> declare the arm64 single kernel image policy to be obsolete.
>>
>> --verbose please.:)   I am sure you're right, but I don't understand
>> the link between the two.
>
> To start making KVM/arm64 modular, you'd have to build it such as
> there is no support for the nVHE hypervisor anymore. Which would mean
> two different configs (one that can only work with VHE, and one for
> the rest) and contradicts the current single kernel image policy.

Ah okay, I interpreted the policy as "it's possible to build a single 
kernel image but it would be possible to build an image for a subset of 
the features as well".

In that case you could have one config that can work either with or 
without VHE (and supports y/n) and one config that can only work with 
VHE (and supports y/m/n).  The code to enter VHE EL2 would of course 
always be builtin.

> It is bad enough that we have to support 3 sets of page sizes.
> Doubling the validation space for the sake of being able to unload KVM
> seems a dubious prospect.

It's not even a configuration that matches kconfig very well, since it 
does have a way to build something *only as a module*, but not a way to 
build something only as built-in.

That said, if you had the possibility to unload/reload KVM, you'll 
quickly become unable to live without it. :)

Paolo