linux-kernel - Re: [PATCH 0/2] KVM: x86: Fix incorrect VM-exit profiling

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Date:   Wed, 18 May 2022 15:34:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Suleiman Souhlal <suleiman@...gle.com>
Cc:     Wei Zhang <zhanwei@...gle.com>, Sangwhan Moon <sxm@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        Linux Kernel <linux-kernel@...r.kernel.org>,
        Jing Zhang <jingzhangos@...gle.com>,
        David Matlack <dmatlack@...gle.com>
Subject: Re: [PATCH 0/2] KVM: x86: Fix incorrect VM-exit profiling

On Wed, May 18, 2022, Suleiman Souhlal wrote:
> On Tue, May 17, 2022 at 4:30 AM Wei Zhang <zhanwei@...gle.com> wrote:
> >
> > > Please don't top-post.  From https://people.kernel.org/tglx/notes-about-netiquette:
> >
> > Ah, I didn't know this should be avoided. Thanks for the info!
> >
> > > My preference would be to find a more complete, KVM-specific solution.  The
> > > profiling stuff seems like it's a dead end, i.e. will always be flawed in some
> > > way.  If this cleanup didn't require a new hypercall then I wouldn't care, but
> > > I don't love having to extend KVM's guest/host ABI for something that ideally
> > > will become obsolete sooner than later.
> >
> > I also feel that adding a new hypercall is too much here. A
> > KVM-specific solution is definitely better, and the eBPF based
> > approach you mentioned sounds like the ultimate solution (at least for
> > inspecting exit reasons).
> >
> > +Suleiman What do you think? The on-going work Sean described sounds
> > promising, perhaps we should put this patch aside for the time being.
> 
> I'm ok with that.
> That said, the advantage of the current solution is that it already
> exists and is very easy to use, by anyone, without having to write any
> code. The proposed solution doesn't sound like it will be as easy.

My goal/hope is to make the eBPF approach just as easy by providing/building a
library of KVM eBPF programs in tools/ so that doing common things like profiling
VM-Exits doesn't require reinventing the wheel.  And those programs could be used
(and thus implicitly tested) by KVM selftests to verify the kernel functionality.