Re: [PATCH] Allow RDTSC and RDTSCP from userspace

[Andy Lutomirski]

> On Apr 25, 2020, at 12:10 PM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> 
> ïOn 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.
> 

I assume the race you mean is:

#VC
Immediate NMI before IST gets shifted
#VC

Kaboom.

How are you dealing with this?  Ultimately, I think that NMI will need to turn off IST before engaging in any funny business. Let me ponder this a bit.

> 
> Regards,
> 
>    Joerg
>