Re: [PATCH v3 09/23] hw/uefi: add var-service-core.c

[Alexander Graf]

On 12.02.25 16:18, Gerd Hoffmann wrote:
>    Hi,
>
> > > Yes.  Knowing both physical and virtual address works only for memory
> > > you allocated yourself before ExitBootServices.  So you can't pass on
> > > pointers from the OS, you have to copy the data to a buffer where you
> > > know the physical address instead.  Yes, some overhead.  Should still
> > > be much faster than going to pio transfer mode ...
> > MacOS takes over the full physical address map past ExitBootServices: Your
> > code no longer has VA access to random code
> That is totally fine.  EFI drivers must register everything they need as
> runtime memory.  Anything else can be unmapped by the OS when calling
> EFI services.
>
> > and it literally memcpy()'s all preserved (virtual available) code and
> > data to different physical addresses.
> Uhm.  I have my doubts this copying behavior is blessed by the UEFI spec.


I don't remember anything in the spec prohibiting it.


> > You simply have nothing that is all of 1) RAM (mapped as cacheable on
> > ARM), 2) known VA 3) known PA.
> Bummer.
>
> > So we really really need a fallback mechanism that works without DMA
> > :).
> On arm it should be relatively simple to move the buffer to device
> memory.  Just place one more region on the platform bus, advertise
> address + size via device tree, done.


That will bring back all issues with cached vs non-cached memory 
accesses, no? So edk2 will always access that memory as device memory 
which means it bypasses the cache, while QEMU will access it through the 
cache. So that buffer would need to actually be MMIO memory I suppose?


> Not sure how to do that best on x86 though.  Find 64k unused address
> space over ioapic?  Do we have enough free space there?  And how
> future-proof would that be?


I'm not worried yet about where we place that memory, but more about 
ensuring that we actually have a working path to access it. We can 
always find space in the PCI hole, as long as we properly advertise it 
to all stakeholders via ACPI and memory map.


Alex