linux-kernel - Re: [PATCH RFC 2/6] x86/hyper-v: add a function to read both TSC and TSC page value simulateneously

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Message-ID: <87o9neoo63.fsf@vitty.brq.redhat.com>
Date:   Mon, 04 Dec 2017 10:08:52 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Stephen Hemminger <stephen@...workplumber.org>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        kvm@...r.kernel.org,
        Radim Krčmář 
        <rkrcmar@...hat.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, devel@...uxdriverproject.org,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RFC 2/6] x86/hyper-v: add a function to read both TSC and TSC page value simulateneously

Paolo Bonzini <pbonzini@...hat.com> writes:

> On 01/12/2017 18:29, Stephen Hemminger wrote:
>>> +static inline u64 hv_read_tsc_page_tsc(const struct ms_hyperv_tsc_page *tsc_pg,
>>> +				       u64 *cur_tsc)
>>> +{
>>> +	*cur_tsc = rdtsc();
>>> +
>>> +	return cur_tsc;
>> Why do return and setting by reference. Looks like an ugly API.
>
> This is the fallback implementation for !CONFIG_HYPERV_TSCPAGE, which
> explains why it's ugly, but why is it needed at all (or it could just
> BUG())?
>

It is not needed indeed, the intention was just to avoid '#if
IS_ENABLED(CONFIG_HYPERV)' in kvm code. I can replace it with BUG() or
return U64_MAX;

-- 
  Vitaly