linux-kernel - Re: [PATCH v4 30/30] KVM: selftests: Add test to populate a VM with the max possible guest mem

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Message-ID: <YifF8QrNxDX4qU4J@google.com>
Date:   Tue, 8 Mar 2022 21:09:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Marc Zyngier <Marc.Zyngier@....com>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v4 30/30] KVM: selftests: Add test to populate a VM with
 the max possible guest mem

On Tue, Mar 08, 2022, Paolo Bonzini wrote:
> On 3/3/22 20:38, Paolo Bonzini wrote:
> > On x86, perform two passes with a MMU context reset between each pass to
> > coerce KVM into dropping all references to the MMU root, e.g. to emulate
> > a vCPU dropping the last reference.  Perform both passes and all
> > rendezvous on all architectures in the hope that arm64 and s390x can gain
> > similar shenanigans in the future.
> 
> Did you actually test aarch64 (not even asking about s390 :))?  For now
> let's only add it for x86.

Nope, don't you read my cover letters?  :-D

  The selftest at the end allows populating a guest with the max amount of
  memory allowed by the underlying architecture.  The most I've tested is
  ~64tb (MAXPHYADDR=46) as I don't have easy access to a system with
  MAXPHYADDR=52.  The selftest compiles on arm64 and s390x, but otherwise
  hasn't been tested outside of x86-64.  It will hopefully do something
  useful as is, but there's a non-zero chance it won't get past init with
  a high max memory.  Running on x86 without the TDP MMU is comically slow.

> > +			TEST_ASSERT(nr_vcpus, "#DE");
> 
> srsly? :)

LOL, yes.  IIRC I added that because I screwed up computing nr_vcpus and my
test did nothing useful :-)