From mboxrd@z Thu Jan  1 00:00:00 1970
From: James Hogan <james.hogan@imgtec.com>
Subject: Re: [PATCH 11/21] MIPS: KVM: Rewrite count/compare timer emulation
Date: Fri, 25 Apr 2014 22:05:42 +0100
Message-ID: <1591723.zAIKIVVUH7@radagast>
References: <1398439204-26171-1-git-send-email-james.hogan@imgtec.com> <1398439204-26171-12-git-send-email-james.hogan@imgtec.com> <535A94A5.1040200@gmail.com>
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Cc: David Daney <ddaney.cavm@gmail.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Gleb Natapov <gleb@kernel.org>, kvm@vger.kernel.org,
        Ralf Baechle <ralf@linux-mips.org>,
        Sanjay Lal <sanjayl@kymasys.com>
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Hi David,

On Friday 25 April 2014 10:00:21 David Daney wrote:
> > +	/* Frequency of timer in Hz */
> > +	uint32_t count_hz;
> 
> We are currently running with timer frequencies of over 2GHz, so the
> width of this variable is close to the limit.

Interesting. Do you have 64-bit Count/Compare registers or just accept that 
you'll wrap after only 2 seconds? (it seems like a fairly short maximum idle 
time, though perhaps not important in the grand scheme of things)

> Your follow-on patch exports this to user-space as part of the KVM ABI.
> 
> I would suggest, at least for the user-space ABI, to make this a 64-bit
> wide value.

Yes, it's 64-bit wide in the userspace API, so it should be future proof. 
Currently it refuses to accept a value higher than 1GHz for numerical reasons 
though. If this same timer code is to be adapted for sleeping VZ guests too 
with those sorts of timer frequencies then it makes sense to ensure it can 
handle higher frequencies (since OTOH VZ doesn't give you control over the 
guest timer frequency, just the offset).

It's not strictly required for T&E though so I don't think it should be a 
blocking factor.

Thanks
James

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