Hi Peter, Thank you for looking into this. On 9/5/2023 9:17 PM, Peter Zijlstra wrote: > On Mon, Sep 04, 2023 at 09:53:34AM +0000, Manali Shukla wrote: > >> Note that, since IBS registers are swap type C [2], the hypervisor is >> responsible for saving and restoring of IBS host state. Hypervisor >> does so only when IBS is active on the host to avoid unnecessary >> rdmsrs/wrmsrs. Hypervisor needs to disable host IBS before saving the >> state and enter the guest. After a guest exit, the hypervisor needs to >> restore host IBS state and re-enable IBS. > > Why do you think it is OK for a guest to disable the host IBS when > entering a guest? Perhaps the host was wanting to profile the guest. > 1. Since IBS registers are of swap type C [1], only guest state is saved and restored by the hardware. Host state needs to be saved and restored by hypervisor. In order to save IBS registers correctly, IBS needs to be disabled before saving the IBS registers. 2. As per APM [2], "When a VMRUN is executed to an SEV-ES guest with IBS virtualization enabled, the IbsFetchCtl[IbsFetchEn] and IbsOpCtl[IbsOpEn] MSR bits must be 0. If either of these bits are not 0, the VMRUN will fail with a VMEXIT_INVALID error code." This is enforced by hardware on SEV-ES guests when VIBS is enabled on SEV-ES guests. 3. VIBS is not enabled by default. It can be enabled by an explicit qemu command line option "-cpu +ibs". Guest should be invoked without this option when host wants to profile the guest. [1] https://bugzilla.kernel.org/attachment.cgi?id=304653 AMD64 Architecture Programmer’s Manual, Vol 2, Appendix B. Layout of VMCB, Table B-2. VMCB Layout, State Save Area Table B-4. VMSA Layout, State Save Area for SEV-ES [2] https://bugzilla.kernel.org/attachment.cgi?id=304653 AMD64 Architecture Programmer’s Manual, Vol 2, Section 15.38, Instruction-Based Sampling Virtualization > Only when perf_event_attr::exclude_guest is set is this allowed, > otherwise you have to respect the host running IBS and you're not > allowed to touch it. > > Host trumps guest etc.. - Manali
