Linux Virtualization - Feb 2017 - [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

Stephen Hemminger <sthemmin at microsoft.com> writes:
> Why not use existing seqlock's?
>
To be honest I don't quite understand how we could use it -- the
sequence locking here is done against the page updated by the
hypersior, we're not creating new structures (so I don't understand how
we could use struct seqcount which we don't have) but I may be
misunderstanding something.

BTW, I just occured to me that I should've probably put the TSC reading
code to mshyperv.h and use it from both vDSO and read_hv_clock_tsc() --
what do you thing?

[snip]

-- 
  Vitaly

On Fri, 10 Feb 2017, Vitaly Kuznetsov wrote:
> Stephen Hemminger <sthemmin at microsoft.com> writes:
> 
> > Why not use existing seqlock's?
> >
> 
> To be honest I don't quite understand how we could use it -- the
> sequence locking here is done against the page updated by the
> hypersior, we're not creating new structures (so I don't understand
how
> we could use struct seqcount which we don't have) but I may be
> misunderstanding something.
You can't use seqlock, but you might be able to use seqcount. Though I
doubt it given the 0 check ....

Thanks,

	tglx

Since sequence count algorithm is done by hypervisor, better to not reuse
seqcount.
Still concerned that the code is racy.

-----Original Message-----
From: Thomas Gleixner [mailto:tglx at linutronix.de] 
Sent: Friday, February 10, 2017 4:28 AM
To: Vitaly Kuznetsov <vkuznets at redhat.com>
Cc: Stephen Hemminger <sthemmin at microsoft.com>; x86 at kernel.org; Andy
Lutomirski <luto at amacapital.net>; Ingo Molnar <mingo at
redhat.com>; H. Peter Anvin <hpa at zytor.com>; KY Srinivasan <kys
at microsoft.com>; Haiyang Zhang <haiyangz at microsoft.com>; Dexuan
Cui <decui at microsoft.com>; linux-kernel at vger.kernel.org; devel at
linuxdriverproject.org; virtualization at lists.linux-foundation.org
Subject: Re: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

On Fri, 10 Feb 2017, Vitaly Kuznetsov wrote:
> Stephen Hemminger <sthemmin at microsoft.com> writes:
> 
> > Why not use existing seqlock's?
> >
> 
> To be honest I don't quite understand how we could use it -- the 
> sequence locking here is done against the page updated by the 
> hypersior, we're not creating new structures (so I don't understand
> how we could use struct seqcount which we don't have) but I may be 
> misunderstanding something.
You can't use seqlock, but you might be able to use seqcount. Though I doubt
it given the 0 check ....

Thanks,

	tglx