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Re: [Qemu-devel] Help on TLB Flush

From: Alexander Graf
Subject: Re: [Qemu-devel] Help on TLB Flush
Date: Thu, 12 Feb 2015 15:45:29 +0100

> On 12.02.2015, at 15:35, Mark Burton <address@hidden> wrote:
> TLB Flush:
> We have spent a few days on this issue, and still haven’t resolved the best 
> path.
> Our solution seems to work, most of the time, but we still have some strange 
> issues - so I want to check that what we are proposing has a chance of 
> working.
> Our plan is to allow all CPU’s to continue. Potentially one CPU will want to 
> write to the TLBs. Subsequent to the write, it requests a TLB Flush.

Local or global? For local TLB flushes you don't notify the other CPUs at all. 
For global ones, the semantics of the call usually dictate atomicity.

> 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.

FWIW TLBs are always CPU local. When there's a "global TLB flush" instruction, 
it pretty much does stall the CPU, notifies the others to also flush their 
TLBs, waits and then continues.

If this really does become a performance bottleneck (which I doubt it does, 
almost nobody except x86 does global flushes), you can also do some nasty hacky 
tricks, such as (atomically) change the valid bit in remote CPUs TLB entries. 
But really only do this as a last resort if the clean version doesn't perform 


> This means - there is a theoretical period of time when one CPU is writing to 
> the TLBs while other CPU’s are executing.  Our belief is that this has to be 
> handled by software anyway, and this should not be an issue from Qemu’s point 
> of view. 
> The alternative would be to force all other CPU’s to exit before writing the 
> TLB’s - this is both expensive and very painful to organise (as we get into 
> horrid deadlocks whichever way we turn)…
> We’d appreciate some thoughts on this...
> Cheers
> Mark.
>        +44 (0)20 7100 3485 x 210
>  +33 (0)5 33 52 01 77x 210
>       +33 (0)603762104
>       mark.burton

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