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 11:43:32AM +0100, 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?

Natural alignment is more efficient.

> 
> 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.)

Right. As FW_CFG_DATA_SIZE is currently 1 byte, it's already naturally
aligned, and the macro definition I have above actually doesn't change
anything (which is why I gave the overkill alert). However if
FW_CFG_DATA_SIZE was to change, then the natural alignment could be lost.

> 
> The full register block is 3 bytes wide. Is that a problem?

No, it's fine as is, and the FW_CFG_SIZE_ALIGNED would leave it 3 bytes
wide too. FW_CFG_SIZE_ALIGNED only adds future-proofing. However it
really is overkill as the chance that FW_CFG_DATA_SIZE will change is nil.

> 
> Thanks
> Laszlo
> 
> > 
> >> +    g_free(nodename);
> >> +}
> >> +
> >>  static void *machvirt_dtb(const struct arm_boot_info *binfo, int *fdt_size)
> >>  {
> >>      const VirtBoardInfo *board = (const VirtBoardInfo *)binfo;
> >> @@ -604,6 +623,8 @@ static void machvirt_init(MachineState *machine)
> >>       */
> >>      create_virtio_devices(vbi, pic);
> >>  
> >> +    create_fw_cfg(vbi);
> >> +
> >>      vbi->bootinfo.ram_size = machine->ram_size;
> >>      vbi->bootinfo.kernel_filename = machine->kernel_filename;
> >>      vbi->bootinfo.kernel_cmdline = machine->kernel_cmdline;
> >> -- 
> >> 1.8.3.1
> >>
>