Messages in this thread | | | From | Vitaly Kuznetsov <> | Subject | Re: [PATCH v10 30/39] KVM: selftests: Hyper-V PV TLB flush selftest | Date | Mon, 03 Oct 2022 15:01:07 +0200 |
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Sean Christopherson <seanjc@google.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
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