Enabling interrupts in QEMU TPM TIS

[Jason Gunthorpe]

On Thu, Jun 25, 2020 at 06:48:09PM -0400, Stefan Berger wrote:
> On 6/25/20 5:26 PM, Stefan Berger wrote:
> > On 6/25/20 1:28 PM, Jason Gunthorpe wrote:
> > > On Thu, Jun 25, 2020 at 10:56:43AM -0400, Stefan Berger wrote:
> > > > Hello!
> > > > 
> > > >   I want to enable IRQs now in QEMU's TPM TIS device model and I
> > > > need to work
> > > > with the following patch to Linux TIS. I am wondering whether
> > > > the changes
> > > > there look reasonable to you? Windows works with the QEMU modifications
> > > > as-is, so maybe it's a bug in the TIS code (which I had not run into
> > > > before).
> > > > 
> > > > 
> > > > The point of the loop I need to introduce in the interrupt
> > > > handler is that
> > > > while the interrupt handler is running another interrupt may
> > > > occur/be posted
> > > > that then does NOT cause the interrupt handler to be invoked again but
> > > > causes a stall, unless the loop is there.
> > > That seems like a qemu bug, TPM interrupts are supposed to be level
> > > interrupts, not edge.
> > 
> > 
> > Following this document here the hardware may choose to support
> > different types of interrutps:
> > 
> > https://trustedcomputinggroup.org/wp-content/uploads/PC-Client-Specific-Platform-TPM-Profile-for-TPM-2p0-v1p04_r0p37_pub-1.pdf
> > 
> > 
> > Table 23. Edge falling or rising, level low or level high.
> > 
> > So with different steps in the driver causing different types of
> > interrupts, we may get into such situations where we process some
> > interrupt 'reasons' but then another one gets posted, I guess due to
> > parallel processing.
> 
> 
> Another data point: I had the TIS driver working on IRQ 5 (festeoi) without
> the introduction of this loop. There are additional bits being set while the
> interrupt handler is running, but the handler deals with them in the next
> invocation. On IRQ 13 (edge), it does need the loop since the next interrupt
> handler invocation is not happening. 

A loop like that is never the correct way to handle edge interrupts.

I don't think the tpm driver was ever designed for edge, so most
likely the structure and order of the hard irq is not correct.

Jason