linux-kernel - Re: [PATCH v10 30/39] KVM: selftests: Hyper-V PV TLB flush selftest

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Date:   Mon, 3 Oct 2022 16:00:04 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Michael Kelley <mikelley@...rosoft.com>,
        Siddharth Chandrasekaran <sidcha@...zon.de>,
        Yuan Yao <yuan.yao@...ux.intel.com>,
        Maxim Levitsky <mlevitsk@...hat.com>,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v10 30/39] KVM: selftests: Hyper-V PV TLB flush selftest

On Mon, Oct 03, 2022, Sean Christopherson wrote:
> On Mon, Oct 03, 2022, Vitaly Kuznetsov wrote:
> > Sean Christopherson <seanjc@...gle.com> writes:
> > > And if you really need a udelay() and not a
> > > usleep(), IMO it's worth adding exactly that instead of throwing NOPs at the CPU.
> > > E.g. aarch64 KVM selftests already implements udelay(), so adding an x86 variant
> > > would move us one step closer to being able to use it in common tests.
> > 
> > ... so yes, I think we need a delay. The problem with implementing
> > udelay() is that TSC frequency is unknown. We can get it from kvmclock
> > but setting up kvmclock pages for all selftests looks like an
> > overkill. Hyper-V emulation gives us HV_X64_MSR_TSC_FREQUENCY but that's
> > not generic enough. Alternatively, we can use KVM_GET_TSC_KHZ when
> > creating a vCPU but we'll need to pass the value to guest code somehow.
> > AFAIR, we can use CPUID.0x15 and/or MSR_PLATFORM_INFO (0xce) or even
> > introduce a PV MSR for our purposes -- or am I missing an obvious "easy"
> > solution?
> 
> I don't think you're missing anything.  Getting the value into the guest is the
> biggest issue.
> 
> Vishal is solving a similar problem where the guest needs to know the "native"
> hypercall.  We can piggyback that hook to do KVM_GET_TSC_KHZ there during VM
> creation, and then simply define udelay()'s behavior to always operate on the
> "default" frequency.  I.e. if a test wants to change the frequency _and_ use
> udelay() _and_ cares about the precision of udelay(), then that test can go write
> its own code.

Forgot to connect the dots: https://lore.kernel.org/all/YzsC4ibDqGh5qaP9@google.com