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UEFI Memory Descriptor Definition
Posted: Fri Dec 11, 2020 11:35 am
by adamfc2000
The
wiki article gives the structure as follows:
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typedef struct {
UINT32 Type; // EFI_MEMORY_TYPE, Field size is 32 bits followed by 32 bit pad
UINT32 Pad;
EFI_PHYSICAL_ADDRESS PhysicalStart; // Field size is 64 bits
EFI_VIRTUAL_ADDRESS VirtualStart; // Field size is 64 bits
UINT64 NumberOfPages; // Field size is 64 bits
UINT64 Attribute; // Field size is 64 bits
} EFI_MEMORY_DESCRIPTOR;
The UEFI spec I
found (page 160) online gives the definition (without the second UINT32 as padding):
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typedef struct {
UINT32 Type;
EFI_PHYSICAL_ADDRESS PhysicalStart;
EFI_VIRTUAL_ADDRESS VirtualStart;
UINT64 NumberOfPages;
UINT64 Attribute;
} EFI_MEMORY_DESCRIPTOR;
Maybe an error in the wiki?
Re: UEFI Memory Descriptor Definition
Posted: Fri Dec 11, 2020 1:22 pm
by BenLunt
Hi,
adamfc2000 wrote:Maybe an error in the wiki?
No, not necessarily. The members of the structure are aligned per their size. For example, in 64-bit mode, the 8-byte member:
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EFI_PHYSICAL_ADDRESS PhysicalStart
will be aligned on the next 8-byte alignment, which is four bytes past where the "Type" member left off.
The first block of code is just accounting for this. The second block of code is assuming that the compiler was told to self align each member.
In my opinion, the first block of code is best, not only because it accounts for this, but is much more "self documenting" along with the comments given.
Ben
Re: UEFI Memory Descriptor Definition
Posted: Fri Dec 11, 2020 5:39 pm
by zaval
UINT64 will be aligned not only in 64 bit mode, in 32 bit too. it's not the error in the wiki. UEFI specification is full of such misaligned gotchas (EFI_LOADED_IMAGE_PROTOCOL structure is a striking example). It's just a "leave it to the compiler to handle" kind of attitude. I, when was writing my own headers for UEFI, inserted padding members explicitly. hopefully, I did that right.
Re: UEFI Memory Descriptor Definition
Posted: Fri Dec 11, 2020 7:03 pm
by kzinti
I remember running into this very issue with Intel's UEFI headers where the definition of EFI_MEMORY_DESCRIPTOR wouldn't compile properly on GCC (it didn't have the padding field).
I fixed the issue by adding that uint32_t padding after "Type".
I also added this in my source code to make sure such problems don't happen again:
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// Intel's UEFI header do not properly define EFI_MEMORY_DESCRIPTOR for GCC.
// This check ensures that it is.
static_assert(offsetof(EFI_MEMORY_DESCRIPTOR, PhysicalStart) == 8);
This fix isn't needed when using MS packing rules as PhysicalStart will be 64-bits aligned (VS, Mingw) but is required with GCC in 32 bits mode at least.