RE: [PATCH 2/2] x86/vdso: Add VCLOCK_HVCLOCK vDSO clock read method

[Stephen Hemminger]

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@xxxxxxxxxxxxx] 
Sent: Friday, February 10, 2017 4:28 AM
To: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
Cc: Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; x86@xxxxxxxxxx; Andy Lutomirski <luto@xxxxxxxxxxxxxx>; Ingo Molnar <mingo@xxxxxxxxxx>; H. Peter Anvin <hpa@xxxxxxxxx>; KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>; Dexuan Cui <decui@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxxxx; virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxxxxx> 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