Linux Virtualization - Apr 2020 - [PATCH 40/70] x86/sev-es: Setup per-cpu GHCBs for the runtime handler

On Wed, Apr 22, 2020 at 06:33:13PM -0700, Bo Gan wrote:
> 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?
Thanks for you analysis. I fixed it (verified by Mike) by using
early_set_memory_decrypted() instead of set_memory_decrypted(). I still
wonder why I didn't see that issue on my kernel. It has
DEBUG_PAGEALLOC=y set, but it is not enabled by default and I also
didn't pass the command-line parameter.

Regards,

	Joerg