Re: [PATCH v1 5/6] riscv/virt: Add the PFlash CFI01 device

[Laszlo Ersek]

On 09/24/19 11:32, Philippe Mathieu-Daudé wrote:
> On 9/23/19 11:46 PM, Peter Maydell wrote:
>> On Fri, 20 Sep 2019 at 23:23, Alistair Francis <address@hidden> wrote:
>>> On Thu, Sep 19, 2019 at 10:15 PM Bin Meng <address@hidden> wrote:
>>>> I don't think we should mirror what is used on ARM virt board to
>>>> create 2 flash for sifive_u. For ARM virt, there are 2 flashes because
>>>> they need distinguish secure and non-secure. For sifive_u, only one is
>>>> enough.
>>>
>>> I went back and forward about 1 or 2. Two seems more usable as maybe
>>> someone wants to include two pflash files? The Xilinx machine also has
>>> two so I'm kind of used to 2, but I'm not really fussed.
> 
> The Xilinx machine has 2 because it matches the hardware.
> 
>> One of the reasons for having 2 on the Arm board (we do this
>> even if we're not supporting secure vs non-secure) is that
>> then you can use one for a fixed read-only BIOS image (backed
>> by a file on the host filesystem shared between all VMs), and
>> one backed by a read-write per-VM file providing permanent
>> storage for BIOS environment variables. Notably UEFI likes to
>> work this way, but the idea applies in theory to other
>> boot loader or BIOSes I guess.
> 
> IIRC Laszlo's explanation, the only reason it is that way is because the
> pflash_cfi01 model still doesn't implement sector locking. At the OVMF
> emerged from EDK2, to have a safe ROM vs DATA storage it was decided to
> use 2 different flashes.
> When I understood when this config layout started, I suggested Laszlo to
> use a real ROM to store the OVMF CODE since it is pointless to do
> firmware upgrade in virtualized environment, but he said it was too late
> to change the design.

Right, at that point I couldn't see how "-bios" *plus* "-pflash" could
have worked. In chronological order (for OVMF anyway), there was -bios,
then -pflash (R/W), then (with some QEMU changes) two -pflash flags (R/O
+ R/W, which OVMF wouldn't tell apart from the single R/W -pflash).

Thanks
Laszlo

> 
> If you don't plan to run UEFI "Capsule Update" on your Virt board, I
> suggest using memory_region_init_rom() with your firmware CODE, and a
> parallel/SPI flash for the data VARStore.
> 
>> I would suggest also checking with Markus that your code
>> for instantiating the flash devices follows the current
>> recommendations so the backing storage can be configured
>> via -blockdev. (This is a fairly recent change from June or
>> so; current-in-master virt and sbsa boards provide an example
>> of doing the right thing, I think.)
>>
>> thanks
>> -- PMM
>>