Hi Gavin, I think this series would benefit from being in smaller pieces. I got lost in patch 4 for quite a while. Suggestion on where to do that in patch 4. On 18/08/2020 02:13, Gavin Shan wrote: > There are two stages of page fault. The guest kernel is responsible > for handling stage one page fault, while the host kernel is to take > care of the stage two page fault. When page fault is triggered because > of stage two page fault, the guest is suspended until the requested > memory (page) is populated. Sometimes, the cost to populate the requested > page isn't cheap and can take hundreds of milliseconds in extreme > cases. This impacts the overall guest's performance. You really need to use postcopy live migration to justify these changes. Otherwise the story here is "over-commited hosts suck", which I don't think anyone cares about. > This series introduces the feature (asynchronous page fault) to resolve > the issue and improve the guest's performance. It depends on the series > to support SDEI virtualization and refactoring SDEI client driver. SDEI gives you an NMI ... which you use to set a TIF flag. This can only work reliably for user-space. So much so that you have code in the hypervisor to only deliver the NMI ... when in user-space. The only reason you would need an NMI is to interrupt interrupts-masked code. Linux can't reschedule when this is the case. I can only conclude, you really don't need an NMI here. Why couldn't we use an IRQ here, it would be a lot simpler? ... the reason is the arm architecture can't guarantee us that we take the irq when there is also a stage2 fault for the first instruction. I reckon we can work around this in the hypervisor: https://lore.kernel.org/r/20201023165108.15061-1-james.morse@xxxxxxx My problem with SDEI is, you don't really need an NMI, and it creates extra in-kernel state that has to be migrated. I think having this state in the kernel complicates the user-space handling of SIGBUS_MCEERR_AO signals that don't get delivered to a vCPU thread. Thanks, James _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
