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Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        X86 ML <x86@kernel.org>,
        Platform Driver <platform-driver-x86@vger.kernel.org>,
        linux-sgx@vger.kernel.org, nhorman@redhat.com,
        npmccallum@redhat.com, "Ayoun, Serge" <serge.ayoun@intel.com>,
        shay.katz-zamir@intel.com,
        Haitao Huang <haitao.huang@linux.intel.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Svahn, Kai" <kai.svahn@intel.com>, mark.shanahan@intel.com,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Darren Hart <dvhart@infradead.org>,
        Andy Shevchenko <andy@infradead.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>
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From:   Dave Hansen <dave.hansen@intel.com>
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Message-ID: <18800fdc-a2e1-39a3-9ee5-0065865ea052@intel.com>
Date:   Mon, 17 Dec 2018 12:15:47 -0800
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Sender: linux-kernel-owner@vger.kernel.org
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On 12/17/18 12:10 PM, Andy Lutomirski wrote:
>> There's no 'struct page' for enclave memory as it stands.  That means no
>> page cache, and that means there's no 'struct address_space *mapping' in
>> the first place.
>>
>> Basically, the choice was made a long time ago to have SGX's memory
>> management live outside the core VM.  I've waffled back and forth on it,
>> but I do still think this is the right way to do it.
> AFAICS a lack of struct page isn't a problem.  The core code seems to
> understand that address_space objects might cover non-struct-page
> memory.  Morally, enclave memory is a lot like hot-unpluggable PCI
> space.

Yeah, this is true.  The existing code seems to make it all the way from
unmap_mapping_range() down to zap_page_range() without 'struct page'.

Overall, I think what Andy is saying here is that an open(/dev/sgx)
should give you a "unique" enclave fd.  That fd can end up mapped into
one or more processes either via fork() or the other ways fds end up
getting handed around.  mmap() of this fd would be *required* to be
MAP_SHARED.  That means you don't need to support COW, and the semantics
are the same as any other MAP_SHARED mapping: children and parents and
anybody mmap()'ing it must all coordinate.

This sounds interesting at least.  It might lead to an unholy mess in
the driver, or it might be a great cleanup.  But, it does sound like
something that would both potentially simplify the semantics and the
implementation.