Shared memory regions mapped as private

[MatthiasGr]

Hello everyone,

While working with the SVSM code base, I encountered a small but hard to debug (at least for me :)).
When mapping a memory region using PageTable::map_region_4k, setting the shared flag should allocate the resulting memory region as shared addresses. The function correctly sets the address bit to do that, but then calls PageTable::map_4k, which always forces a private page address. The 2m versions of these functions have the same issue.

Since both functions are public APIs, I am not sure if just removing the call to make_private_address in map_4k would be safe. I unfortunately could not get rust-analyzer to a point where it reliably reports symbol usage across the project. One way to address this would be to just inline the call to map_{4k,2m} in the respective map_region function.

Regards
Matthias

[00xc]

When mapping a memory region using PageTable::map_region_4k, setting the shared flag should allocate the resulting memory region as shared addresses. The function correctly sets the address bit to do that, but then calls PageTable::map_4k, which always forces a private page address. The 2m versions of these functions have the same issue.

I think you are meant to be using something like a PerCPUPageMappingGuard. It has this exact pattern, which as you note, is kind of inconsistent:

svsm/kernel/src/mm/ptguards.rs

Lines 114 to 117 in fbe0c83

	pgtable.map_4k(vaddr, paddr, flags)?;
	if shared {
	pgtable.set_shared_4k(vaddr)?;
	}

But in general I think we should fix PageTable::map_4k to take a bool parameter, just like PageTablePart::map_4k(), so that we have a single call, instead of having to fix the PTE afterwards, like the code above does. I can prepare a PR for that.

I unfortunately could not get rust-analyzer to a point where it reliably reports symbol usage across the project.

If you change your working directory to the kernel/ path you should get it working. This is sadly a shortcoming of having multiple build targets in the same workspace.

[MatthiasGr]

Thank you for your quick reply. Also thanks for the tip regarding rust analyzer, I'll try that the next time.