How do we use UEFI SetVirtualAddressMap

[devc1]

Why do OS loaders use SetVirtualAddressMap, and how to map virtual memory inside UEFI bootloader to load a higher half kernel.

[Octocontrabass]

Why do OS loaders use SetVirtualAddressMap,

Do they? Linux seems to assume the loader won't use it, but I have no idea what Windows does.

Anyway, the intention is to make it easier for the OS to call runtime services by allowing the OS to choose their virtual addresses instead of requiring identity mapping.

and how to map virtual memory inside UEFI bootloader to load a higher half kernel.

1. Use boot services to load your kernel and build your page tables
2. Call ExitBootServices()
3. Switch to your newly-built page tables
4. Jump to your kernel

[devc1]

That's what I was going to do anyway. I will copy the code to map the page tables in the bootloader, there is no need for a new page table since the kernel only uses the upperhalf (>128 TB).

SetVirtualaddrMap makes runtime drivers switch from physical to virtual addressing using ConvertPointer for variables, I did it after ExitBootServices but I don't think that I will really need change the mappings, so if a user mode program needs to check the time, it will syscall and simply Ill switch to the kernel's page table and runtime services will work well as I think.

[devc1]

Now I'm stuck to link my assembly file to the kernel (to get the value of cr3). I'm using EDK2

[Octocontrabass]

if a user mode program needs to check the time, it will syscall and simply Ill switch to the kernel's page table and runtime services will work well as I think.

Be careful with that. Two user mode programs running on two CPUs might syscall at the same time, but most runtime services can only run on one CPU at a time (and not at the same time as any other runtime services).

Now I'm stuck to link my assembly file to the kernel (to get the value of cr3). I'm using EDK2

Kernel or bootloader? You shouldn't be using EDK2 for your kernel. (If you're using EDK2 with GCC, you can use GCC's inline assembly. I don't know how to link external assembly with EDK2.)

[devc1]

It's just the bootloader, and btw thank you for the tip, I didn't think of that.

[devc1]

I found out that UEFI sets it page tables as read only so I'm gonna need to make a new page table.