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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 23, 2022 at 12:10:09PM +0100, Eric Auger wrote:
> 
> 
> On 11/23/22 10:30, Michael S. Tsirkin wrote:
> > 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?
> 
> I don't think there is any need and since it violates the alignment
> check in VFIO we discard the region from DMA mapped ones.
> 
> Thanks
> 
> Eric

If that's all then we could just check alignment -
why are we bothering with a tpm specific hack?


> >
> >>>>> ---
> >>>>>   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 */