Re: [Qemu-devel] selecting VIRTIO_INPUT and VIRTIO_VGA

[Laszlo Ersek <lersek@redhat.com>]

On 07/26/15 12:17, Peter Maydell wrote:
> On 26 July 2015 at 10:31, Laszlo Ersek <lersek@redhat.com> wrote:
>> On 07/25/15 11:49, Gerd Hoffmann wrote:
>>> virtio-vga in vga-compat mode uses a framebuffer with the usual dirty
>>> tracking logic in pci bar 0 (simliar to stdvga).  Which is exactly the
>>> thing causing the cache coherency issues on aarch64 if I understand
>>> things correctly.
>>
>> Yes. :(
> 
> The question for cache-coherency of an emulated framebuffer with
> KVM is simply "does the guest set up this region of physical address
> space as Normal Cacheable memory, or does it set it up as Device
> or some other non-cacheable memory attribute type?". Whether the
> framebuffer is part of a PCI BAR or shared with "guest RAM" doesn't
> matter for this. (Of course it may matter for the guest if the
> guest makes assumptions about what kind of mapping it needs to
> use for any PCI BAR.)
> 
> [NB: this is a statement about the current situation of kernel and
> QEMU code and doesn't take account of any attempts we might make to
> make things work better.]
> 
>>> Programming (modesetting) works without legacy vga io
>>> ports, you can use the mmio regs in pci bar 1 instead (applies to both
>>> virtio-vga and stdvga btw), and QemuVideoDxe actually uses the mmio bar.
>>
>> True.
>>
>> But, as a side point, let me talk a bit about the outb() function in
>> OvmfPkg/QemuVideoDxe/Driver.c. It (very correctly for a UEFI_DRIVER
>> module!) uses PciIo->Io.Write() to write to IO ports.
>>
>> Now, the PciIo protocol implementation is platform independent. In
>> practice it forwards IO space accesses to the EFI_PCI_ROOT_BRIDGE_IO
>> protocol. And *that* one is platform-dependent.
>>
>> For x86 virtual machines, those accesses are turned into IO port
>> accesses. However, the EFI_PCI_ROOT_BRIDGE_IO implementation in
>> ArmVirtPkg/PciHostBridgeDxe/, which is built into AAVMF and runs on the
>> "virt" machtype, maps the IO space and the IO port accesses to a special
>> (fake) MMIO range of 64K "ports".
>>
>> In QEMU this memory region corresponds to VIRT_PCIE_PIO, in
>> "hw/arm/virt.c". See create_pcie():
>>
>>     hwaddr base_pio = vbi->memmap[VIRT_PCIE_PIO].base;
>>
>>     ...
>>
>>     /* Map IO port space */
>>     sysbus_mmio_map(SYS_BUS_DEVICE(dev), 2, base_pio);
> 
> This seems to me to be confusing two things:
>  (1) x86-style IO ports as accessed via inb/outb insns
>  (2) PCI IO space
> 
> For the ARM boards we certainly support the latter (it's part
> of the PCI spec and mapping PCI IO space to some window in
> physical address space is the usual approach for CPUs which
> don't have x86's special-purpose io instructions). This isn't
> going to go away because you need it for dealing with PCI
> devices that have IO BARs. To access registers in this window
> the guest needs to program the PCI card's IO BAR to make it
> appear somewhere in the window.
> 
> We don't support legacy VGO io ports, which on x86 just
> always exist whether the guest programs a PCI BAR or not,
> and which have fixed legacy port numbers.

I was wrong to call the PCI IO space "fake", sorry about that, and
thanks for the correction. But, whatever i said about the guest
firmware, should be correct. "outb" in the QemuVideoDxe source is just a
familiar name that Jordan chose in 2011. However, it correctly calls
PciIo->Io.Write(), which on x86 (ultimately, through several layers)
results in ioport instructions, and on arm results in MMIO accesses.

I may not have understood the full theoretical background of
FDT_PCI_RANGE_IOPORT, but I think the firmware code that puts it to use
is correct.

Thanks
Laszlo