[edk2-devel] [OVMF] What would be the best way to make PcdPciMmio64Size dynamic?

[Boeuf, Sebastien]

On Tue, 2023-01-17 at 15:38 +0100, kraxel at redhat.com wrote:
> On Tue, Jan 17, 2023 at 02:10:08PM +0000, Boeuf, Sebastien wrote:
> > Hi,
> > 
> > Looking at the hardcoded limitation provided by PcdPciMmio64Size
> > (32GiB), I was wondering what would be the best approach to make
> > this a
> > bit more dynamic?
> > 
> > I know that for QEMU, the fw_cfg mechanism can be used to override
> > the
> > value of PcdPciMmio64Size, but this isn't something Cloud
> > Hypervisor or
> > other VMMs can implement given they don't support fw_cfg at all.
> > 
> > Would it be acceptable to dynamically compute PcdPciMmio64Size from
> > the
> > physical address space? Given the reason why PcdPciMmio64Size can't
> > be
> > increased is to make sure it can support host with small address
> > space
> > (such as 36 bits), we could introduce a very simple function that
> > would
> > determine the available address space and consider PcdPciMmio64Size
> > being half the size.
> 
> Sure, check master branch, the code is already there.  Should just be
> a
> matter of wiring up the function calls for cloundhv, and you can
> probably call PlatformAddressWidthFromCpuid() with QemuQuirk = false
> unconditionally.

Ah thank you for the pointer, the code is exactly what I had in mind.
I sent a patch to disable the QemuQuirk in case of Cloud Hypervisor,
which then result in the proper address space size to be returned, and
therefore the 64-bit MMIO window to be computed dynamically!

Thanks,
Sebastien

> 
> take care,
>   Gerd
> 

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