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Date:   Mon, 3 Oct 2022 15:47:26 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Vitaly Kuznetsov <vkuznets@redhat.com>
Cc:     kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Michael Kelley <mikelley@microsoft.com>,
        Siddharth Chandrasekaran <sidcha@amazon.de>,
        Yuan Yao <yuan.yao@linux.intel.com>,
        Maxim Levitsky <mlevitsk@redhat.com>,
        linux-hyperv@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v10 30/39] KVM: selftests: Hyper-V PV TLB flush selftest
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On Mon, Oct 03, 2022, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> writes:
> > Anyways, why not do e.g. usleep(1)?  
> 
> I was under the impression that all these 'sleep' functions result in a
> syscall (and I do see TRIPLE_FAULT when I swap my rep_nop() with usleep())
> and here we need to wait in the guest (sender) ...

Oh, duh, guest code.

> > 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.

> I'm thinking about being lazy here and implemnting a Hyper-V specific
> udelay through HV_X64_MSR_TSC_FREQUENCY (unless you object, of course)
> to avoid bloating this series beyond 46 patches it already has.

I'm totally fine being even lazier here and just using a loop of nops, but with a
different function name and a TODO (I completely forgot this was guest code when
making the usleep() suggestion).  Then we can clean up the TODO via udelay() in a
follow-up series.