Re: [PATCH] Allow RDTSC and RDTSCP from userspace

[Joerg Roedel]

On Sat, Apr 25, 2020 at 11:15:35AM -0700, Andy Lutomirski wrote:
> shift_ist is gross.  What's it for?  If it's not needed, I'd rather
> not use it, and I eventually want to get rid of it for #DB as well.

The #VC handler needs to be able to nest, there is no way around that
for various reasons, the two most important ones are:

	1. The #VC -> NMI -> #VC case. #VCs can happen in the NMI
	   handler, for example (but not exclusivly) for RDPMC.

	2. In case of an error the #VC handler needs to print out error
	   information by calling one of the printk wrappers. Those will
	   end up doing IO to some console/serial port/whatever which
	   will also cause #VC exceptions to emulate the access to the
	   output devices.

Using shift_ist is perfect for that, the only problem is the race
condition with the NMI handler, as shift_ist does not work well with
exceptions that can also trigger within the NMI handler. But I have
taken care of that for #VC.


Regards,

	Joerg