Hi Kirill,

On Thu, Jul 22, 2021 at 10:51:30PM +0300, Kirill A. Shutemov wrote:

Okay, below is my first take on the topic.

Thanks, I havn't looked deeply into the patch yet, but will do so
tomorrow and reply separatly.

I ended up combing your idea with bitmap with PageOffline(): early boot
code uses bitmap, but on page allocator init I mark unaccepted pages with
PageOffline(). This way page allocator need to touch the bitmap only when
it steps on PageOffline() which shouldn't be often once things settle
after boot.

I still need to understand the benefit of having this information in the
memmap, but I also don't object to it. For AMD-SNP the bitmap needs to
stay around at least, unless there is another way to implement
kexec/kdump.

One bit in the bitmap represents 2M region. Any unaligned chunks gets
accepted when we construct the bitmap. This way one 4K page can represent
64 GiB of physical address space.

Yeah, a 2MB chunk size makes sense when it comes to how much we validate
at once. I think it will be good choice for AMD too. On the other side
there is a need for SNP to track shared pages on a 4k granularity. There
are a couple of shared (or at least not valid) pages (GHCB, #HV shared page,
VMSA page) per vCPU which are 4k in size. Oh, and then there is the
.bss_decrypted section, which is also not 2M aligend.

In case of kexec/kdump this information needs to be passed on to the
next kernel.

Regards,

	Jörg