Re: [PATCH 04/11] tpm_crb: use a single read-as-mem/write-as-mmio mapping

[Stefan Berger]

On 7/13/23 10:50, Peter Maydell wrote:
> On Thu, 13 Jul 2023 at 15:18, Stefan Berger <stefanb@linux.ibm.com> wrote:
> > On 7/12/23 23:51, Joelle van Dyne wrote:
> > > On Apple Silicon, when Windows performs a LDP on the CRB MMIO space,
> > > the exception is not decoded by hardware and we cannot trap the MMIO
> > > read. This led to the idea from @agraf to use the same mapping type as
> > > ROM devices: namely that reads should be seen as memory type and
> > > writes should trap as MMIO.

> > > +++ b/hw/tpm/tpm_crb.c
> > > @@ -68,7 +68,6 @@ static const VMStateDescription vmstate_tpm_crb_none = {
> > >        .name = "tpm-crb",
> > >        .pre_save = tpm_crb_none_pre_save,
> > >        .fields = (VMStateField[]) {
> > > -        VMSTATE_UINT32_ARRAY(state.regs, CRBState, TPM_CRB_R_MAX),
> >
> > This has to stay here otherwise we cannot restart VMs from saved state once 
> > QEMU is upgraded.
> >
> > 2023-07-13T14:15:43.997718Z qemu-system-x86_64: Unknown ramblock "tpm-crb-cmd", 
> > cannot accept migration
> > 2023-07-13T14:15:43.997813Z qemu-system-x86_64: error while loading state for 
> > instance 0x0 of device 'ram'
> > 2023-07-13T14:15:43.997841Z qemu-system-x86_64: load of migration failed: 
> > Invalid argument
>
> More generally, for migration compatibility in the other
> direction you need to use memory_region_init_rom_device_nomigrate()
> and make sure you keep migrating the data via this, not
> via the MemoryRegion.
>
> I'm not a super-fan of hacking around the fact that LDP
> to hardware registers isn't supported in specific device
> models, though...

What does this mean for this effort here?

   Stefan

> thanks
> -- PMM