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

[Andrew Jones <drjones@redhat.com>]

On Fri, Nov 28, 2014 at 11:49:48AM +0100, Laszlo Ersek wrote:
> 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.

Actually, on second thought, completely scratch my overkill suggestion.
It's actually wrong to be concerned with it anyway. FW_CFG_DATA_SIZE
doesn't dictate how we should access the data port, the fw-cfg protocol
does, and that says we should access exactly one byte. So, regardless of
the fw-cfg-data size, we'll never have to worry about the data port's
alignment, as we'll never access more than one byte from it.

> 
> Thanks!
> Laszlo