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Re: [Qemu-devel] [RFC PATCH v2 2/3] utils: Add cpuinfo helper to fetch /

From: Will Deacon
Subject: Re: [Qemu-devel] [RFC PATCH v2 2/3] utils: Add cpuinfo helper to fetch /proc/cpuinfo
Date: Tue, 10 May 2016 11:24:04 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

On Mon, May 09, 2016 at 01:44:11PM +0000, Catalin Marinas wrote:
> On Mon, May 09, 2016 at 12:21:08PM +0100, Peter Maydell wrote:
> > On 9 May 2016 at 11:59, Suzuki K Poulose <address@hidden> wrote:
> > > Well, we have been waiting for a use case, like this, before we merge
> > > the series.
> > 
> > This isn't a great strategy for moving people away from things
> > you'd like them to avoid like parsing /proc/cpuinfo, because typically
> > userspace app writers are not very interested in coding to facilities
> > which don't exist yet, and will prefer to make do with what's actually
> > present in the kernel today... You need to provide the improved API,
> > and then it needs to get out into kernel versions in distros and
> > otherwise, and only then are you likely to get app developers who
> > will start to say "this is useful".
> The problem is that the way kernel people think the API may be improved
> does not always match the use-cases required by app writers. One example
> here is exposing MIDR via MRS emulation, we know there are problems with
> big.LITTLE and the only clear answer I got so far is that we ignore such
> configurations. We don't even have a way to tell user space that this is
> a heterogeneous CPU configuration, unless we add another HWCAP bit
> specifically for this (or the opposite: HWCAP_HOMOGENEOUS_CPUS).

Personally, I think we should expose big.LITTLE as-is to userspace. That
is, if you execute an mrs instruction you'll get whichever core the
emulation happens to run on. This might even be useful to things like
pinned threadpools w/ userspace schedulers sitting on top.

> That said, I'm perfectly fine with exposing:
>       /sys/devices/system/cpu/cpu$ID/identification/
>                                                       \- midr
>                                                       \- revidr
> I had the wrong impression that we already merged this part but Suzuki
> just pointed out to me that it's not.

Yes, there are use-cases for this interface as well. I don't think it's
a choice between one or the other and I firmly believe we need both (the
sysfs and mrs code).

> I think our 4.7-rc1 tree is pretty much frozen to new features now,
> though the sysfs patch is relatively small (I'll let Will comment):

The merge window opens in less than a week, so it's fixes only atm.


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