linux-kernel - Re: [PATCH] x86/sgx: Allow exposing EDECCSSA user leaf function to KVM guest

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Message-ID: <5efaad269ff8dfcbecff311dd42b019530995049.camel@intel.com>
Date:   Wed, 17 Aug 2022 09:49:25 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "linux-sgx@...r.kernel.org" <linux-sgx@...r.kernel.org>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>
CC:     "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "haitao.huang@...ux.intel.com" <haitao.huang@...ux.intel.com>
Subject: Re: [PATCH] x86/sgx: Allow exposing EDECCSSA user leaf function to
 KVM guest

On Wed, 2022-07-27 at 23:54 +1200, Kai Huang wrote:
> Coupled with the new SGX attribute to allow one enclave to receive the
> AEX notification, SGX also adds a new EDECCSSA user leaf function to
> facilitate the AEX notification handling.  The new EDECCSSA is
> enumerated via CPUID(EAX=0x12,ECX=0x0):EAX[11].
> 
> Besides Allowing reporting the new AEX-notify attribute to KVM guests,
> also allow reporting the new EDECCSSA user leaf function to KVM guests
> so the guest can fully utilize the AEX-notify mechanism.
> 
> Introduce a new X86 CPU feature flag for the new EDECCSSA, and report it
> in KVM's supported CPUIDs so the userspace hypervisor (i.e. Qemu) can
> enable it for the guest.
> 
> Note there's no additional enabling work required to allow guest to use
> the new EDECCSSA.  KVM is not able to trap ENCLU anyway.
> 
> More background about how do AEX-notify and EDECCSSA work:
> 
> The new Asynchronous Exit (AEX) notification mechanism (AEX-notify)
> allows one enclave to receive a notification in the ERESUME after the
> enclave exit due to an AEX.  EDECCSSA is a new SGX user leaf function
> (ENCLU[EDECCSSA]) to facilitate the AEX notification handling.
> 
> SGX maintains a Current State Save Area Frame (CSSA) for each enclave
> thread.  When AEX happens, the enclave thread context is saved to the
> CSSA and the CSSA is increased by 1.  For a normal ERESUME which doesn't
> deliver AEX notification, it restores the saved thread context from the
> previously saved SSA and decreases the CSSA.  If AEX-notify is enabled
> for one enclave, the ERESUME acts differently.  Instead of restoring the
> saved thread context and decreasing the CSSA, it acts like EENTER which
> doesn't decrease the CSSA but establishes a clean slate thread context
> at the CSSA for the enclave to handle the notification.  After some
> handling, the enclave must discard the "new-established" SSA and switch
> back to the previous saved SSA (upon AEX).  Otherwise, the enclave will
> run out of SSA space upon further AEXs and eventually fail to run.
> 
> To solve this problem, the new EDECCSSA essentially decreases the CSSA.
> It can be used by the enclave notification handler to switch back to the
> previous saved SSA when needed, i.e. after it handles the notification.
> 
> Signed-off-by: Kai Huang <kai.huang@...el.com>
> ---
> Hi Dave,
> 
> This is the patch you requested.  Feel free to merge.
> 
> This patch isn't tested and needs KVM maintainers' review.

Sorry this patch has bugs.  I forgot to handle the new X86_FEATURE_SGX_EDECCSSA
bit in scattered CPUID handling (and forgot to add it to the CPUID dependency
table). Will send v2 after testing.


-- 
Thanks,
-Kai