On Wed, Feb 24, 2021, Ashish Kalra wrote: > # Samples: 19K of event 'kvm:kvm_hypercall' > # Event count (approx.): 19573 > # > # Overhead Command Shared Object Symbol > # ........ ............... ................ ......................... > # > 100.00% qemu-system-x86 [kernel.vmlinux] [k] kvm_emulate_hypercall > > Out of these 19573 hypercalls, # of page encryption status hcalls are 19479, > so almost all hypercalls here are page encryption status hypercalls.
Oof. > The above data indicates that there will be ~2% more Heavyweight VMEXITs > during SEV guest boot if we do page encryption status hypercalls > pass-through to host userspace. > > But, then Brijesh pointed out to me and highlighted that currently > OVMF is doing lot of VMEXITs because they don't use the DMA pool to minimize > the C-bit toggles, > in other words, OVMF bounce buffer does page state change on every DMA > allocate and free. > > So here is the performance analysis after kernel and initrd have been > loaded into memory using grub and then starting perf just before booting the > kernel. > > These are the performance #'s after kernel and initrd have been loaded into > memory, > then perf is attached and kernel is booted : > > # Samples: 1M of event 'kvm:kvm_userspace_exit' > # Event count (approx.): 1081235 > # > # Overhead Trace output > # ........ ........................ > # > 99.77% reason KVM_EXIT_IO (2) > 0.23% reason KVM_EXIT_MMIO (6) > > # Samples: 1K of event 'kvm:kvm_hypercall' > # Event count (approx.): 1279 > # > > So as the above data indicates, Linux is only making ~1K hypercalls, > compared to ~18K hypercalls made by OVMF in the above use case. > > Does the above adds a prerequisite that OVMF needs to be optimized if > and before hypercall pass-through can be done ? Disclaimer: my math could be totally wrong. I doubt it's a hard requirement. Assuming a conversative roundtrip time of 50k cycles, those 18K hypercalls will add well under a 1/2 a second of boot time. If userspace can push the roundtrip time down to 10k cycles, the overhead is more like 50 milliseconds. That being said, this does seem like a good OVMF cleanup, irrespective of this new hypercall. I assume it's not cheap to convert a page between encrypted and decrypted. Thanks much for getting the numbers!
