On Fri, Dec 1, 2023 at 10:05 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote: > > On Fri, Nov 10, 2023, Jacky Li wrote: > > The cache flush operation in sev guest memory reclaim events was > > originally introduced to prevent security issues due to cache > > incoherence and untrusted VMM. However when this operation gets > > triggered, it causes performance degradation to the whole machine. > > > > This cache flush operation is performed in mmu_notifiers, in particular, > > in the mmu_notifier_invalidate_range_start() function, unconditionally > > on all guest memory regions. Although the intention was to flush > > cache lines only when guest memory was deallocated, the excessive > > invocations include many other cases where this flush is unnecessary. > > > > This RFC proposes using the mmu notifier event to determine whether a > > cache flush is needed. Specifically, only do the cache flush when the > > address range is unmapped, cleared, released or migrated. A bitmap > > module param is also introduced to provide flexibility when flush is > > needed in more events or no flush is needed depending on the hardware > > platform. > > I'm still not at all convinced that this is worth doing. We have clear line of > sight to cleanly and optimally handling SNP and beyond. If there is an actual > use case that wants to run SEV and/or SEV-ES VMs, which can't support page > migration, on the same host as traditional VMs, _and_ for some reason their > userspace is incapable of providing reasonable NUMA locality, then the owners of > that use case can speak up and provide justification for taking on this extra > complexity in KVM. Hi Sean, Jacky and I were looking at some cases like mmu_notifier calls triggered by the overloaded reason "MMU_NOTIFY_CLEAR". Even if we turn off page migration etc, splitting PMD may still happen at some point under this reason, and we will never be able to turn it off by tweaking kernel CONFIG options. So, I think this is the line of sight for this series. Handling SNP could be separate, since in SNP we have per-page properties, which allow KVM to know which page to flush individually. Thanks. -Mingwei
