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@xxxxxxxxx> > --- > 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
