From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stephen Hemminger <stephen@networkplumber.org>,
        Stephen Hemminger <sthemmin@microsoft.com>, kvm@vger.kernel.org,
        Radim =?utf-8?B?S3LEjW0=?= =?utf-8?B?w6HFmQ==?= 
        <rkrcmar@redhat.com>,
        Haiyang Zhang <haiyangz@microsoft.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        devel@linuxdriverproject.org, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH RFC 2/6] x86/hyper-v: add a function to read both TSC and TSC page value simulateneously
References: <20171201131321.918-1-vkuznets@redhat.com>
        <20171201131321.918-3-vkuznets@redhat.com>
        <20171201092904.30d9831f@xeon-e3>
        <77674d0f-f4c6-ad47-1984-ea040e256501@redhat.com>
Date: Mon, 04 Dec 2017 10:08:52 +0100
In-Reply-To: <77674d0f-f4c6-ad47-1984-ea040e256501@redhat.com> (Paolo
        Bonzini's message of "Fri, 1 Dec 2017 18:52:48 +0100")
Message-ID: <87o9neoo63.fsf@vitty.brq.redhat.com>
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Paolo Bonzini <pbonzini@redhat.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