Re: [PATCH v5 1/2] sysemu: tpm: Add a stub function for TPM_IS_CRB

[Michael S. Tsirkin]

On Wed, Nov 23, 2022 at 09:18:39AM +0100, Eric Auger wrote:
> Hi,
> 
> On 11/23/22 07:36, Michael S. Tsirkin wrote:
> > On Fri, May 06, 2022 at 09:47:52AM -0400, Stefan Berger wrote:
> >>
> >> On 5/6/22 09:25, Eric Auger wrote:
> >>> In a subsequent patch, VFIO will need to recognize if
> >>> a memory region owner is a TPM CRB device. Hence VFIO
> >>> needs to use TPM_IS_CRB() even if CONFIG_TPM is unset. So
> >>> let's add a stub function.
> >>>
> >>> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>> Suggested-by: Cornelia Huck <cohuck@redhat.com>
> >> Reviewed-by: Stefan Berger <stefanb@linnux.ibm.com>
> > ... and now in 7.2 vdpa needs a dependency on tpm too, what a hack :(
> > And what exactly is it about TPM CRB that everyone needs to
> > know about it and skip it? The API does not tell ...
> An excerpt of one reply I made at that time:
> 
> The spec (CG PC Client Platform TPM Profile (PTP)
>     Specification Family “2.0” Level 00 Revision 01.03 v22, page 100) 
> says that the command/response data "may be defined as large as 3968",
> which is (0x1000 - 0x80), 0x80 being the size of the control struct.
> so the size of the region logically is less than a 4kB page, hence our
> trouble.
> 
> We learnt in the past Windows driver has some stronger expectation wrt
> memory mapping. I don't know if those latter would work if we were to
> enlarge the window by some tricks.
> 
> https://trustedcomputinggroup.org/wp-content/uploads/Mobile-Command-Response-Buffer-Interface-v2-r12-Specification_FINAL2.pdf
> says
> 
> "
> Including the control structure, the three memory areas comprise the
> entirety of the CRB. There are no constraints on how those three memory
> areas are provided. They can all be in system RAM, or all be in device
> memory, or any combination.
> 
> Thanks
> 
> Eric

So we put it in system RAM then? But why isn't DMA there allowed?

> >
> >>> ---
> >>>   include/sysemu/tpm.h | 6 ++++++
> >>>   1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/include/sysemu/tpm.h b/include/sysemu/tpm.h
> >>> index 68b2206463c..fb40e30ff60 100644
> >>> --- a/include/sysemu/tpm.h
> >>> +++ b/include/sysemu/tpm.h
> >>> @@ -80,6 +80,12 @@ static inline TPMVersion tpm_get_version(TPMIf *ti)
> >>>   #define tpm_init()  (0)
> >>>   #define tpm_cleanup()
> >>>
> >>> +/* needed for an alignment check in non-tpm code */
> >>> +static inline Object *TPM_IS_CRB(Object *obj)
> >>> +{
> >>> +     return NULL;
> >>> +}
> >>> +
> >>>   #endif /* CONFIG_TPM */
> >>>
> >>>   #endif /* QEMU_TPM_H */
> >>