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

From: Vitaly Kuznetsov
Date: Mon Oct 03 2022 - 09:01:21 EST

Sean Christopherson <seanjc@xxxxxxxxxx> 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"

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.