linux-kernel - Re: [Part1 PATCH v4 15/17] percpu: introduce DEFINE_PER_CPU

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Date:   Tue, 19 Sep 2017 08:50:20 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...e.de>
Cc:     brijesh.singh@....com, linux-kernel@...r.kernel.org,
        x86@...nel.org, kvm@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Arnd Bergmann <arnd@...db.de>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>, linux-arch@...r.kernel.org
Subject: Re: [Part1 PATCH v4 15/17] percpu: introduce
 DEFINE_PER_CPU_UNENCRYPTED

Hi Boris,

On 09/19/2017 05:39 AM, Borislav Petkov wrote:
...

>> @@ -815,6 +825,7 @@
>>   	. = ALIGN(cacheline);						\
>>   	*(.data..percpu)						\
>>   	*(.data..percpu..shared_aligned)				\
>> +	PERCPU_UNENCRYPTED_SECTION					\
>>   	VMLINUX_SYMBOL(__per_cpu_end) = .;
> 
> So looking at this more: I'm wondering if we can simply reuse the
> PER_CPU_SHARED_ALIGNED_SECTION definition which is for shared per-CPU
> sections. Instead of introducing a special section which is going to be
> used only by SEV, practically.
> 
> Because "shared" also kinda implies that it is shared by multiple agents
> and those agents can just as well be guest and hypervisor. And then that
> patch is gone too.
> 
> Hmmm...?
> 

"..shared_aligned" section does not start and end with page-size alignment.
Since the C-bit works on PAGE_SIZE alignment hence the "..unencrypted" section
starts and ends with page-size alignment. The closest I can find is
"..page_aligned" but again it does not end with page-size alignment.

Additionally, since we clear the C-bit from unencrypted section hence we
should avoid overloading the existing section -- we don't want to expose more
than we wish.