Re: [Qemu-devel] [PATCH v2 4/5] tpm: add CRB device

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Jan 19, 2018 at 04:56:31PM -0500, Stefan Berger wrote:
> On 01/19/2018 01:42 PM, Eduardo Habkost wrote:
> > On Fri, Jan 19, 2018 at 12:10:03PM -0500, Stefan Berger wrote:
> > > On 01/19/2018 09:11 AM, Marc-André Lureau wrote:
> > > > tpm_crb is a device for TPM 2.0 Command Response Buffer (CRB)
> > > > Interface as defined in TCG PC Client Platform TPM Profile (PTP)
> > > > Specification Family “2.0” Level 00 Revision 01.03 v22.
> > > > 
> > > > The PTP allows device implementation to switch between TIS and CRB
> > > > model at run time, but given that CRB is a simpler device to
> > > > implement, I chose to implement it as a different device.
> > > > 
> > > > The device doesn't implement other locality than 0 for now (my laptop
> > > > TPM doesn't either, so I assume this isn't so bad)
> > > > 
> > > > The command/reply memory region is statically allocated after the CRB
> > > > registers address TPM_CRB_ADDR_BASE + sizeof(struct crb_regs) (I
> > > > wonder if the BIOS could or should allocate it instead, or what size
> > > > to use, again this seems to fit well expectations)
> > > I removed this last sentence now. It's at the right location.
> > > 
> > > > The PTP doesn't specify a particular bus to put the device. So I added
> > > > it on the system bus directly, so it could hopefully be used easily on
> > > > a different platform than x86. Currently, it fails to init on piix,
> > > > because error_on_sysbus_device() check. The check may be changed in a
> > > > near future, see discussion on the qemu-devel ML.
> > > I think this has to be solved. So I remove these last 2 sentences. I'll have
> > > to wait until that other patch series from Eduard is merged since it doesn't
> > > start yet.
> > The series was just merged to master.  It's possible to make a
> > machine accept the new device using
> > machine_class_allow_dynamic_sysbus_dev(), now.
> 
> I saw that.
> > 
> > However, is it really necessary to make it a sysbus device?
> > Having bus-less devices was not possible in the past, but it is
> > possible today.
> > 
> What I don't like about it is the fact that I would have to use q35 if we
> only extend that machine type to allow this sysbus device. What is the
> reason that dynamic sysbus devices have to explicitly be allowed? If we
> don't need to limit this device to a certain machine type that may be more
> user friendly.

Most sysbus devices don't work unless they have additional
supporting machine code to wire them at the right addresses.
Devices that work without extra machine code (like *-iommu) are
the exception.

I think the last time I saw an explanation for this was at:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg439549.html

-- 
Eduardo