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

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Message-ID: <87wn9h9i3w.fsf@redhat.com>
Date:   Mon, 03 Oct 2022 15:01:07 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <seanjc@...gle.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

Sean Christopherson <seanjc@...gle.com> writes:

> On Wed, Sep 21, 2022, Vitaly Kuznetsov wrote:

...

>> +}
>> +
>> +/* Delay */
>> +static inline void rep_nop(void)
>
> LOL, rep_nop() is a hilariously confusing function name.  "REP NOP" is "PAUSE",
> and for whatever reason the kernel proper use rep_nop() as the function name for
> the wrapper.  My reaction to the MFENCE+rep_nop() below was "how the hell does
> MFENCE+PAUSE guarantee a delay?!?".

Well, at least you got the joke :-)

>
> 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) ...

> 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'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.

...

-- 
Vitaly