Hi Peter, On 9/7/2023 1:26 AM, Peter Zijlstra wrote: > On Wed, Sep 06, 2023 at 09:08:25PM +0530, Manali Shukla wrote: >> 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. > > I'm not sure I'm fluent in virt speak (in fact, I'm sure I'm not). Is > the above saying that a host can never IBS profile a guest? Host can profile a guest with IBS if VIBS is disabled for the guest. This is the default behavior. Host can not profile guest if VIBS is enabled for guest. > > Does the current IBS thing assert perf_event_attr::exclude_guest is set? Unlike AMD core pmu, IBS doesn't have Host/Guest filtering capability, thus perf_event_open() fails if exclude_guest is set for an IBS event. > > I can't quickly find anything :-( Thank you, Manali
