Re: Re: [PATCH 40/70] x86/sev-es: Setup per-cpu GHCBs for the runtime handler

[Bo Gan]

On 4/15/20 8:53 AM, Joerg Roedel wrote:
> Hi Mike,
>
> On Tue, Apr 14, 2020 at 07:03:44PM +0000, Mike Stunes wrote:
> > set_memory_decrypted needs to check the return value. I see it
> > consistently return ENOMEM. I've traced that back to split_large_page
> > in arch/x86/mm/pat/set_memory.c.
>
> I agree that the return code needs to be checked. But I wonder why this
> happens. The split_large_page() function returns -ENOMEM when
> alloc_pages() fails. Do you boot the guest with minal RAM assigned?
>
> Regards,
>
> 	Joerg

I just want to add some context around this. The call path that lead to 
the failure is like the following:

	__alloc_pages_slowpath
	__alloc_pages_nodemask
	alloc_pages_current
	alloc_pages
	split_large_page
	__change_page_attr
	__change_page_attr_set_clr
	__set_memory_enc_dec
	set_memory_decrypted
	sev_es_init_ghcbs
	trap_init   -> before mm_init (in init/main.c)
	start_kernel
	x86_64_start_reservations
	x86_64_start_kernel
	secondary_startup_64

At this time, mem_init hasn't been called yet (which would be called by 
mm_init). Thus, the free pages are still owned by memblock. It's in 
mem_init (x86/mm/init_64.c) that memblock_free_all gets called and free 
pages are released.

During testing, I've also noticed that debug_pagealloc=1 will make the 
issue disappear. That's because with debug_pagealloc=1, 
probe_page_size_mask in x86/mm/init.c will not allow large pages 
(2M/1G). Therefore, no split_large_page would happen. Similarly, if CPU 
doesn't have X86_FEATURE_PSE, there won't be large pages either.

Any thoughts? Maybe split_large_page should get pages from memblock at 
early boot?

Bo