Re: [PATCH v4 1/2] tpm: CRB: Use ram_device for "tpm-crb-cmd" region

[Peter Maydell]

On Tue, 8 Feb 2022 at 15:08, Eric Auger <eric.auger@redhat.com> wrote:
>
> Representing the CRB cmd/response buffer as a standard
> RAM region causes some trouble when the device is used
> with VFIO. Indeed VFIO attempts to DMA_MAP this region
> as usual RAM but this latter does not have a valid page
> size alignment causing such an error report:
> "vfio_listener_region_add received unaligned region".
> To allow VFIO to detect that failing dma mapping
> this region is not an issue, let's use a ram_device
> memory region type instead.

This seems like VFIO's problem to me. There's nothing
that guarantees alignment for memory regions at all,
whether they're RAM, IO or anything else.

> +    s->crb_cmd_buf = qemu_memalign(qemu_real_host_page_size,
> +                                HOST_PAGE_ALIGN(CRB_CTRL_CMD_SIZE));
> +
>      memory_region_init_io(&s->mmio, OBJECT(s), &tpm_crb_memory_ops, s,
>          "tpm-crb-mmio", sizeof(s->regs));
> -    memory_region_init_ram(&s->cmdmem, OBJECT(s),
> -        "tpm-crb-cmd", CRB_CTRL_CMD_SIZE, errp);
> +    memory_region_init_ram_device_ptr(&s->cmdmem, OBJECT(s), "tpm-crb-cmd",
> +                                      CRB_CTRL_CMD_SIZE, s->crb_cmd_buf);
> +    vmstate_register_ram(&s->cmdmem, dev);
>
>      memory_region_add_subregion(get_system_memory(),
>          TPM_CRB_ADDR_BASE, &s->mmio);
> @@ -309,12 +315,25 @@ static void tpm_crb_realize(DeviceState *dev, Error 
> **errp)
>      qemu_register_reset(tpm_crb_reset, dev);
>  }

As QEMU code goes, this seems much worse than what it replaces.
To have a memory region backed by RAM and migrated in the
usual way, memory_region_init_ram() is the right thing.

thanks
-- PMM