Linux Virtualization - Apr 2020 - [PATCH] Allow RDTSC and RDTSCP from userspace

> On Apr 25, 2020, at 12:10 PM, Joerg Roedel <joro at 8bytes.org>
wrote:
> 
> ?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.
> 
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
>

On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski
wrote:
> 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.
Right, I dealt with that by unconditionally shifting/unshifting the #VC IST
entry
in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
one of the IST stacks to be unused during nesting, but that is fine. The
stack memory for #VC is only allocated when SEV-ES is active (in an
SEV-ES VM).

Regards,

	Joerg

On Sat, Apr 25, 2020 at 1:23 PM Joerg Roedel <joro at 8bytes.org>
wrote:
>
> On Sat, Apr 25, 2020 at 12:47:31PM -0700, Andy Lutomirski wrote:
> > 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.
>
> Right, I dealt with that by unconditionally shifting/unshifting the #VC IST
entry
> in do_nmi() (thanks to Davin Kaplan for the idea). It might cause
> one of the IST stacks to be unused during nesting, but that is fine. The
> stack memory for #VC is only allocated when SEV-ES is active (in an
> SEV-ES VM).
Blech.  It probably works, but still, yuck.  It's a bit sad that we
seem to be growing more and more poorly designed happens-anywhere
exception types at an alarming rate.  We seem to have #NM, #MC, #VC,
#HV, and #DB.  This doesn't really scale.

--Andy