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Date:   Sat, 22 Dec 2018 02:01:45 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>, X86 ML <x86@kernel.org>,
        Platform Driver <platform-driver-x86@vger.kernel.org>,
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        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,
        Suresh Siddha <suresh.b.siddha@intel.com>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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        Andy Shevchenko <andy@infradead.org>,
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        <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v17 18/23] platform/x86: Intel SGX driver
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On Fri, Dec 21, 2018 at 10:28:09AM -0800, Sean Christopherson wrote:
> > Why would you want to pass EPC through user space to KVM rather than
> > KVM allocating it through kernel interfaces?
> 
> Delegating EPC management to userspace fits better with KVM's existing
> memory ABI.  KVM provides a single ioctl(), KVM_SET_USER_MEMORY_REGION[1],
> that allows userspace to create, move, modify and delete memory regions.
> 
> Skipping over a lot of details, there are essentially three options for
> exposing EPC to a KVM guest:
> 
>  1) Provide a dedicated KVM ioctl() to manage EPC without routing it
>     through KVM_SET_USER_MEMORY_REGION.
> 
>  2) Add a flag to 'struct kvm_userspace_memory_region' that denotes an
>     EPC memory region and mmap() / allocate EPC in KVM.
> 
>  3) Provide an ABI to allocate raw EPC and let userspace manage it like
>     any other memory region.
> 
> Option (1) requires duplicating all of KVM_SET_USER_MEMORY_REGION's
> functionality unless the ioctl() is severly restricted.
> 
> Option (2) is an ugly abuse of KVM_SET_USER_MEMORY_REGION since the EPC
> flag would have completely different semantics than all other usage of
> KVM_SET_USER_MEMORY_REGION.
> 
> Thus, option (3).

OK, thank you for patience explaining this.

> Probably a better question to answer is why provide the ABI through
> /dev/sgx and not /dev/kvm.  IMO /dev/sgx is a more logical way to
> advertise support to userspace, e.g. userspace can simply check if
> /dev/sgx (or /dev/sgx/epc) exists vs. probing a KVM capability.

You have to understand that for a KVM non-expert like me it was really
important to get the context, which you kindly gave. I have never used
KVM's memory management API but now that I know how it works all of this
makes perfect sense. This is not a better question but it is definitely
a good follow up question :-)

I don't really understand you deduction here, however. If SGX was not
supported, why couldn't the hypothetical /dev/kvm functionality just
return an error?

For me it sounds a bit messy that KVM functionality, which is a client
to the SGX functionality, places some of its functionality to the SGX
core.

/Jarkko