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

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Date:   Thu, 23 Apr 2020 13:30:27 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Bo Gan <ganb@...are.com>
Cc:     Mike Stunes <mstunes@...are.com>, Joerg Roedel <joro@...tes.org>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: Re: [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