Why not use existing seqlock's?
-----Original Message-----
From: Thomas Gleixner [mailto:t...@linutronix.de]
Sent: Thursday, February 9, 2017 9:08 AM
To: Vitaly Kuznetsov <vkuzn...@redhat.com>
Cc: x...@kernel.org; Andy Lutomirski <l...@amacapital.net>; Ingo Molnar
<mi...@redhat.com>; H. Peter Anvin <h...@zytor.com>; KY Srinivasan
<k...@microsoft.com>; Haiyang Zhang <haiya...@microsoft.com>; Stephen Hemminger
<sthem...@microsoft.com>; Dexuan Cui <de...@microsoft.com>;
linux-ker...@vger.kernel.org; de...@linuxdriverproject.org;
virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method
On Thu, 9 Feb 2017, Vitaly Kuznetsov wrote:
> +#ifdef CONFIG_HYPERV_TSCPAGE
> +static notrace u64 vread_hvclock(int *mode) {
> + const struct ms_hyperv_tsc_page *tsc_pg =
> + (const struct ms_hyperv_tsc_page *)&hvclock_page;
> + u64 sequence, scale, offset, current_tick, cur_tsc;
> +
> + while (1) {
> + sequence = READ_ONCE(tsc_pg->tsc_sequence);
> + if (!sequence)
> + break;
> +
> + scale = READ_ONCE(tsc_pg->tsc_scale);
> + offset = READ_ONCE(tsc_pg->tsc_offset);
> + rdtscll(cur_tsc);
> +
> + current_tick = mul_u64_u64_shr(cur_tsc, scale, 64) + offset;
> +
> + if (READ_ONCE(tsc_pg->tsc_sequence) == sequence)
> + return current_tick;
That sequence stuff lacks still a sensible explanation. It's fundamentally
different from the sequence counting we do in the kernel, so documentation for
it is really required.
Thanks,
tglx
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