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X-MS-Exchange-Transport-CrossTenantHeadersStamped: SA1PR03MB6531 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 22/03/2023 5:49 pm, Sean Christopherson wrote: > On Mon, Mar 06, 2023, Xin Li wrote: >> Execute "int $2" to handle NMI in NMI caused VM exits when FRED is enabled. >> >> Like IRET for IDT, ERETS/ERETU are required to end the NMI handler for FRED >> to unblock NMI ASAP (w/ bit 28 of CS set). > That's "CS" on the stack correct? Is bit 28 set manually by software, or is it > set automatically by hardware? If it's set by hardware, does "int $2" actually > set the bit since it's not a real NMI? int $2 had better not set it...  This is the piece of state that is intended to cause everything which isn't a real NMI to nest properly inside a real NMI. It is supposed to be set on delivery of an NMI, and act as the trigger for ERET{U,S} to drop the latch. Software is can set it manually in a FRED-frame in order to explicitly unblock NMIs. ~Andrew