Re: [RFC PATCH 0/4] KVM: SEV: Limit cache flush operations in sev guest memory reclaim events

[Sean Christopherson <seanjc@google.com>]

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.