Re: [PATCH 11/21] MIPS: KVM: Rewrite count/compare timer emulation

James Hogan
2014-04-25T21:05:42Z

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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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