edk2 memory map on QEMU

edk2 memory map on QEMU

[Stuart Yoder]

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I am playing around with EDK2 on QEMU with a UEFI shell application and in the app I allocate some memory using gBS->AllocatePool(EfiBootServicesData, ...)

Programmatically accessing the pointer returned works fine, but when I print it, it does not seem to be what I would expect is a valid address.

I've allocated 4GB to the QEMU machine, which I believe starts at 0x40000000.

But, when I print the address returned by AllocatePool the value is "0x39177018".

I thought that all memory was identity mapped where phys=virt, so not sure where the 0x39177018 is coming from.  Trying to dump 0x39177018 from the QEMU console or GDB results in a bad address error.

Thanks,
Stuart

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Re: [edk2-devel] edk2 memory map on QEMU

[Andrew Fish]

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> On Jul 30, 2021, at 12:32 PM, Stuart Yoder <stuart.yoder@arm.com> wrote:
> 
> I am playing around with EDK2 on QEMU with a UEFI shell application and in the app I allocate some memory using gBS->AllocatePool(EfiBootServicesData, ...)
> 
> Programmatically accessing the pointer returned works fine, but when I print it, it does not seem to be what I would expect is a valid address.
> 
> I've allocated 4GB to the QEMU machine, which I believe starts at 0x40000000.
> 

You can run the `memmap` command at the EFI Shell to see the layout. 

> But, when I print the address returned by AllocatePool the value is "0x39177018".
> 

Print != printf on some of the format string so be careful about that….

Thanks,

Andrew Fish

> I thought that all memory was identity mapped where phys=virt, so not sure where the 0x39177018 is coming from.  Trying to dump 0x39177018 from the QEMU console or GDB results in a bad address error.
> Thanks,
> Stuart
> 


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Re: [edk2-devel] edk2 memory map on QEMU

[Stuart Yoder]

On 7/30/21 3:36 PM, Andrew Fish wrote:
> 
> 
>> On Jul 30, 2021, at 12:32 PM, Stuart Yoder <stuart.yoder@arm.com <mailto:stuart.yoder@arm.com>> wrote:
>>
>> I am playing around with EDK2 on QEMU with a UEFI shell application and in the app I allocate some memory using gBS->AllocatePool(EfiBootServicesData, ...)
>>
>> Programmatically accessing the pointer returned works fine, but when I print it, it does not seem to be what I would expect is a valid address.
>>
>> I've allocated 4GB to the QEMU machine, which I believe starts at 0x40000000.
>>
> 
> You can run the `memmap` command at the EFI Shell to see the layout.
> 
>> But, when I print the address returned by AllocatePool the value is "0x39177018".
>>
> 
> Print != printf on some of the format string so be careful about that….
> 

I figured it out.  It was a dumb Print() formatting error that resulted in
the address getting truncated to 8 bytes.

Thanks,
Stuart