Re: [Qemu-devel] [PATCH v3] target-tilegx: Support iret instruction and related special registers

[Chris Metcalf]

On 10/1/2015 10:26 PM, Richard Henderson wrote:
> On 10/02/2015 11:31 AM, Chris Metcalf wrote:
> > It disables interrupts from being delivered.  This means asynchronous
> > interrupts get deferred until ICS is set back to zero, and synchronous
> > interrupts (page fault, etc) cause a double-fault instead. ICS is automatically
> > set on entry to interrupt handlers, so the handler has time to acquire any
> > information about the interrupt from SPRs, and it is expected that ICS is
> > cleared as soon as possible.  ICS can also be used before returning from
> > interrupts if you need to do something like adjust the interrupt mask prior to
> > returning.
>
> Which is all very well and good for supervisor mode... but what's it good for 
> in user mode?

In user-space it is primarily useful for userspace drivers that are receiving 
device interrupts.  You can also configure to receive performance counter 
interrupts in userspace.  The bottom line is that is indeed a rarely-used 
feature.

> I was about to quote you from 2012 (https://lkml.org/lkml/2012/3/30/994):
>
> > In general we want to avoid ever touching memory while within an
> > interrupt critical section, since the page fault path goes through
> > a different path from the hypervisor when in an interrupt critical
> > section, and we carefully decided with tilegx that we didn't need
> > to support this path in the kernel.
>
> Which implies that tilegx userland does nothing at all with ICS, and is in fact 
> unsupported?

This refers to the kernel's own use of the interrupt critical section, i.e. 
having that SPR set while running at PL2.  Each protection level (PL) has its 
own ICS.  So the text you quoted is orthogonal to userspace use of ICS.

--
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com