Re: [PATCH 0/2] Expose KVM API to Linux Kernel

From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Anastassios Nanos <ananos@nubificus.co.uk>
Cc: Wanpeng Li <wanpengli@tencent.com>,
	kvm@vger.kernel.org, Marc Zyngier <maz@kernel.org>,
	Joerg Roedel <joro@8bytes.org>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Borislav Petkov <bp@alien8.de>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Will Deacon <will@kernel.org>,
	kvmarm@lists.cs.columbia.edu, Jim Mattson <jmattson@google.com>
Subject: Re: [PATCH 0/2] Expose KVM API to Linux Kernel
Date: Mon, 18 May 2020 11:18:53 +0200	[thread overview]
Message-ID: <87o8qlvbwi.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <CALRTab-mEYtRG4zQbSGoAri+jg8xNL-imODv=MWE330Hkt_t+Q@mail.gmail.com>

Anastassios Nanos <ananos@nubificus.co.uk> writes:

> Moreover, it doesn't involve *any* mode switch at all while printing
> out the result of the  addition of these two registers -- which I
> guess for a simple use-case like this it isn't much.
> But if we were to scale this to a large number of exits (and their
> respective handling in user-space) that would incur significant
> overhead.

Eliminating frequent exits to userspace when the guest is already
running is absolutely fine but eliminating userspace completely, even
for creation of the guest, is something dubious. To create a simple
guest you need just a dozen of IOCTLs, you'll have to find a really,
really good showcase when it makes a difference. 

E.g. I can imagine the following use-case: you need to create a lot of
guests with the same (or almost the same) memory contents and allocating
and populating this memory in userspace takes time. But even in this
use-case, why do you need to terminate your userspace? Or would it be
possible to create guests from a shared memory? (we may not have
copy-on-write capabilities in KVM currently but this doesn't mean they
can't be added).

Alternatively, you may want to mangle vmexit handling somehow and
exiting to userspace seems slow. Fine, let's add eBPF attach points to
KVM and an API to attach eBPF code there.

I'm, however, just guessing. I understand you may not want to reveal
your original idea for some reason but without us understanding what's
really needed I don't see how the change can be reviewed.

-- 
Vitaly

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