Re: [RFC PATCH 18/18] x86: request TLB flush to slave CPU using NMI

[Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>]

On 2012/06/29 1:38, Avi Kivity wrote:
> On 06/28/2012 09:08 AM, Tomoki Sekiyama wrote:
>> For slave CPUs, it is inapropriate to request TLB flush using IPI.
>> because the IPI may be sent to a KVM guest when the slave CPU is running
>> the guest with direct interrupt routing.
>>
>> Instead, it registers a TLB flush request in per-cpu bitmask and send a NMI
>> to interrupt execution of the guest. Then, NMI handler will check the
>> requests and handles the requests.
> 
> 
> Currently x86's get_user_pages_fast() depends on TLB flushes being held
> up by local_irq_disable().  With this patch, this is no longer true and
> get_user_pages_fast() can race with page table freeing. There are
> patches from Peter Zijlstra to remove this dependency though. 

Thank you for the information. I will check his patches.

> NMIs are
> still slow and fragile when compared to normal interrupts, so this patch
> is somewhat problematic.

OK, always sending NMIs is actually problematic. I should check the
slave core state and send NMIs only when slave guest is
running and NMI is really needed.

Thanks,
-- 
Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>
Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory