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Re: [Qemu-devel] Help on TLB Flush

From: Mark Burton
Subject: Re: [Qemu-devel] Help on TLB Flush
Date: Thu, 12 Feb 2015 16:08:24 +0100

> On 12 Feb 2015, at 16:01, Peter Maydell <address@hidden> wrote:
> On 12 February 2015 at 14:45, Alexander Graf <address@hidden> wrote:
>>> On 12.02.2015, at 15:35, Mark Burton <address@hidden> wrote:
>>> We are proposing to implement this by signalling all other CPU’s
>>> to exit (and requesting they flush before re-starting). In other
>>> words, this would happen asynchronously.
>> For global flushes, give them a pointer payload along with the flush
>> request and tell all cpus to increment it atomically. In your main
>> thread, wait until *ptr == nKickedCpus.
> I bet this will not be the only situation where you want to
> do an "get all other CPUs to do $something and wait til they
> have done so" kind of operation, so some lightweight but generic
> infrastructure for doing that would not be a bad plan. (Similarly
> "get all other CPUs to stop, then I can do $something and let
> the others continue”.)

We tried this - we ended up in knots.
We had 2 CPU’s trying to flush at about the same time, both waiting for the 
We had CPU’s trying to get the global mutex to finish what they were doing, 
while being told to flush, 
We had CPU’s in the global mutex trying to do something that would cause a 
flush… etc....
We had spaghetti with extra Bolognese sauce…

We eventually concluded, yes - in an infinite universe everything is possible, 
but if we could simply do this ‘asynchronously’ then our lives would be a LOT 
e.g.  - ask all CPU’s to “exit and do something” is easy -  wait for them to do 
that is a whole other problem…

Our question is - do we need this ‘sync’ (before the flush), or can we actually 
allow CPU’s to flush themselves asynchronously….



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