[svsm-devel] Allocating 4 contiguous pages and set them shared

[Claudio Carvalho]

On Thu, 2023-11-02 at 13:37 +0100, Carlos López wrote:
> Hi Claudio,

Hi Carlos,

> I'll preface this by saying I'm not completely familiar with the mm
> code, so I could be completely wrong here. That being said, IIUC, page
> order does not have much to do with the page table, but rather with the
> size of the blocks in the free list.
> 
> `MemoryRegion` in src/mm/alloc.rs is initialized with a range of virtual
> addresses (in `root_mem_init()`) and manages that range in blocks of
> determined sizes. It stores a free list for each order, with each free
> list holding blocks of `(1 << order)` pages.
> 
> You'll notice that the code in mm/alloc does not call into the page
> table code at all, meaning that it has no knowledge of page table
> hierarchy (or at least none that I could see). In other words, you
> should be able to give `PageTable::set_shared_4k()` the address of any
> page, and it will (un)set the appropriate bit in the corresponding
> entry. Even if you give it the address of a huge page, the function
> will handle it by calling `PageTable::split_4k()` first.
> 
> In other words, something like this should work:
> 
>      let p = allocate_pages(get_order(4*PAGE_SIZE))?;
>      let pgtable = this_cpu_mut().get_pgtable();
>      for page in (p..p + PAGE_SIZE * 4).step_by(PAGE_SIZE) {
>          pgtable.set_shared_4k(page);
>      }

Yeah, that makes sense. It looks like I'm on the right direction then. I have
some code in my PR #69 that does something like this. I will just go ahead and
move it to the mm.

> 
> Again, maybe I'm wrong here, so maybe someone else can confirm or refute
> my explanation. Jörg wrote most of the mm/alloc code according to git
> blame, but he's out this week.
> 
> PS: with my new `MemoryRegion` type in PR#131, (not to be confused with
> the `MemoryRegion` above), the for loop should be simplified quite a
> bit, as `allocate_pages()` could return a `MemoryRegion<VirtAddr>`,
> which would allow:
> 
>      for page in pages.iter_pages(PageSize::Regular) {
>          pgtable.set_shared_4k(page);
>      }
> 

Nice, I will take a look at it.

Thank you so much Carlos.

Best,
Claudio