Re: [Qemu-devel] [qemu PATCH 2/2] arm: add fw_cfg to "virt" board

[Laszlo Ersek <lersek@redhat.com>]

On 11/28/14 11:43, Laszlo Ersek wrote:
> On 11/28/14 11:38, Andrew Jones wrote:
>> On Fri, Nov 28, 2014 at 12:18:27AM +0100, Laszlo Ersek wrote:
>>> fw_cfg already supports exposure over MMIO (used in ppc/mac_newworld.c,
>>> ppc/mac_oldworld.c, sparc/sun4m.c); we can easily add it to the "virt"
>>> board.
>>>
>>> The mmio register block of fw_cfg is advertized in the device tree. As
>>> base address we pick 0x09020000, which conforms to the comment preceding
>>> "a15memmap": it falls in the miscellaneous device I/O range 128MB..256MB,
>>> and it is aligned at 64KB.
>>>
>>> fw_cfg automatically exports a number of files to the guest; for example,
>>> "bootorder" (see fw_cfg_machine_reset()).
>>>
>>> Signed-off-by: Laszlo Ersek <lersek@redhat.com>
>>> ---
>>>  hw/arm/virt.c | 21 +++++++++++++++++++++
>>>  1 file changed, 21 insertions(+)
>>>
>>> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
>>> index 314e55b..070bd34 100644
>>> --- a/hw/arm/virt.c
>>> +++ b/hw/arm/virt.c
>>> @@ -68,6 +68,7 @@ enum {
>>>      VIRT_UART,
>>>      VIRT_MMIO,
>>>      VIRT_RTC,
>>> +    VIRT_FW_CFG,
>>>  };
>>>  
>>>  typedef struct MemMapEntry {
>>> @@ -107,6 +108,7 @@ static const MemMapEntry a15memmap[] = {
>>>      [VIRT_GIC_CPU] =    { 0x08010000, 0x00010000 },
>>>      [VIRT_UART] =       { 0x09000000, 0x00001000 },
>>>      [VIRT_RTC] =        { 0x09010000, 0x00001000 },
>>> +    [VIRT_FW_CFG] =     { 0x09020000, FW_CFG_SIZE + FW_CFG_DATA_SIZE },
>>>      [VIRT_MMIO] =       { 0x0a000000, 0x00000200 },
>>>      /* ...repeating for a total of NUM_VIRTIO_TRANSPORTS, each of that size */
>>>      /* 0x10000000 .. 0x40000000 reserved for PCI */
>>> @@ -519,6 +521,23 @@ static void create_flash(const VirtBoardInfo *vbi)
>>>      g_free(nodename);
>>>  }
>>>  
>>> +static void create_fw_cfg(const VirtBoardInfo *vbi)
>>> +{
>>> +    hwaddr base = vbi->memmap[VIRT_FW_CFG].base;
>>> +    char *nodename;
>>> +
>>> +    fw_cfg_init(0, 0, base, base + FW_CFG_SIZE);
>>> +
>>> +    nodename = g_strdup_printf("/fw-cfg@%" PRIx64, base);
>>> +    qemu_fdt_add_subnode(vbi->fdt, nodename);
>>> +    qemu_fdt_setprop_string(vbi->fdt, nodename,
>>> +                            "compatible", "fw-cfg,mmio");
>>> +    qemu_fdt_setprop_sized_cells(vbi->fdt, nodename, "reg",
>>> +                                 2, base, 2, FW_CFG_SIZE,
>>> +                                 2, base + FW_CFG_SIZE, 2, FW_CFG_DATA_SIZE);
>>
>> Overkill suggestion alert, but how about defining something like
>>
>> #define FW_CFG_SIZE_ALIGNED \
>>     MIN(QEMU_ALIGN_UP(FW_CFG_SIZE, FW_CFG_DATA_SIZE), \
>>         QEMU_ALIGN_UP(FW_CFG_SIZE, 4))
>>
>> and then using that in your memmap size calculation and fw-cfg-data base
>> address calculation. The only reason I suggest this is because it's hard
>> to tell that fw-cfg-data's address will be naturally aligned without
>> hunting down the definition of FW_CFG_DATA_SIZE. And, if it were to change
>> (which it probably never will), then it may not be.
> 
> Why does it need to be aligned?
> 
> The selector register is aligned at a 64KB boundary (for independent,
> strict reasons).
> 
> The data register is not aligned at all, and -- AFAICS -- it need not
> be, because it's 1 byte wide. (In fact the ARM-specific
> Mmio(Read|Write)XX functions in edk2 enforce natural alignment, and the
> above layout passes without problems.)
> 
> The full register block is 3 bytes wide. Is that a problem?

Hm, I think I get it now. If FW_CFG_DATA_SIZE were to increase, then its
alignment would have to increase as well, and whatever alignment
FW_CFG_SIZE provides might not suffice. So, you'd calculate the natural
alignment, but wouldn't increase it beyond 4.

I do think this is a bit overkill :) but I can do it. Let's wait for
more review comments first.

Thanks!
Laszlo