linux-kernel - Re: [PATCH V4 0/4] KVM: x86: Make bus clock frequency for vAPIC timer configurable

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Message-ID: <ZilAEhUS-mmgjBK8@google.com>
Date: Wed, 24 Apr 2024 10:23:30 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Rick P Edgecombe <rick.p.edgecombe@...el.com>
Cc: "jmattson@...gle.com" <jmattson@...gle.com>, Chao Gao <chao.gao@...el.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "vkuznets@...hat.com" <vkuznets@...hat.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Xiaoyao Li <xiaoyao.li@...el.com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Erdem Aktas <erdemaktas@...gle.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "mlevitsk@...hat.com" <mlevitsk@...hat.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH V4 0/4] KVM: x86: Make bus clock frequency for vAPIC timer configurable

On Wed, Apr 24, 2024, Rick P Edgecombe wrote:
> On Wed, 2024-04-24 at 09:13 -0700, Sean Christopherson wrote:
> > On Tue, Apr 16, 2024, Rick P Edgecombe wrote:
> > > On Thu, 2024-03-21 at 09:37 -0700, Reinette Chatre wrote:
> > > > 
> > > > Summary
> > > > -------
> > > > Add KVM_CAP_X86_APIC_BUS_FREQUENCY capability to configure the APIC
> > > > bus clock frequency for APIC timer emulation.
> > > > Allow KVM_ENABLE_CAPABILITY(KVM_CAP_X86_APIC_BUS_FREQUENCY) to set the
> > > > frequency in nanoseconds. When using this capability, the user space
> > > > VMM should configure CPUID leaf 0x15 to advertise the frequency.
> > > 
> > > Looks good to me and...
> > > Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> > > 
> > > The only thing missing is actually integrating it into TDX qemu patches and
> > > testing the resulting TD. I think we are making a fair assumption that the
> > > problem should be resolved based on the analysis, but we have not actually
> > > tested that part. Is that right?
> > 
> > Please tell me that Rick is wrong, and that this actually has been tested with
> > a TDX guest.  I don't care _who_ tested it, or with what VMM it has been
> > tested, but _someone_ needs to verify that this actually fixes the TDX issue.
> 
> It is in the process of getting a TDX test developed (or rather updated).
> Agreed, it requires verification that it fixes the original TDX issue. That is
> why I raised it.
> 
> Reinette was working on this internally and some iterations were happening, but
> we are trying to work on the public list as much as possible per your earlier
> comments. So that is why she posted it.

I have no problem posting "early", but Documentation/process/maintainer-kvm-x86.rst
clearly states under Testing that:

  If you can't fully test a change, e.g. due to lack of hardware, clearly state
  what level of testing you were able to do, e.g. in the cover letter.

I was assuming that this was actually *fully* tested, because nothing suggests
otherwise.  And _that_ is a problem, e.g. I was planning on applying this series
for 6.10, which would have made for quite the mess if it turns out that this
doesn't actually solve the TDX problem.

> There was at least some level of TDX integration in the past. I'm not sure what
> exactly was tested, but we are going to re-verify it with the latest everything.

Honest question, is it a big lift to re-test the QEMU+KVM TDX changes, e.g. to
verify this new capability actually does what we hope it does?  If testing is a
big lift, what are the pain points?  Or perhaps a better question is, is there
anything we (both upstream people, and end users like Google) can do to make
re-testing less awful?