Re: [PATCH 6/9] KVM: arm64: Split huge pages when dirty logging is enabled

[Sean Christopherson <seanjc@google.com>]

On Tue, Jan 31, 2023, Oliver Upton wrote:
> On Tue, Jan 31, 2023 at 01:18:15AM +0000, Sean Christopherson wrote:
> > On Mon, Jan 30, 2023, Oliver Upton wrote:
> > > I think that Marc's suggestion of having userspace configure this is
> > > sound. After all, userspace _should_ know the granularity of the backing
> > > source it chose for guest memory.
> > > 
> > > We could also interpret a cache size of 0 to signal that userspace wants
> > > to disable eager page split for a VM altogether. It is entirely possible that
> > > the user will want a differing QoS between slice-of-hardware and
> > > overcommitted VMs.
> > 
> > Maybe.  It's also entirely possible that QoS is never factored in, e.g. if QoS
> > guarantees for all VMs on a system are better met by enabling eager splitting
> > across the board.
> > 
> > There are other reasons to use module/kernel params beyond what Marc listed, e.g.
> > to let the user opt out even when something is on by default.  x86's TDP MMU has
> > benefited greatly from downstream users being able to do A/B performance testing
> > this way.  I suspect x86's eager_page_split knob was added largely for this
> > reason, e.g. to easily see how a specific workload is affected by eager splitting.
> > That seems like a reasonable fit on the ARM side as well.
> 
> There's a rather important distinction here in that we'd allow userspace
> to select the page split cache size, which should be correctly sized for
> the backing memory source. Considering the break-before-make rules of
> the architecture, the only way eager split is performant on arm64 is by
> replacing a block entry with a fully populated table hierarchy in one
> operation. AFAICT, you don't have this problem on x86, as the
> architecture generally permits a direct valid->valid transformation
> without an intermediate invalidation. Well, ignoring iTLB multihit :)
> 
> So, the largest transformation we need to do right now is on a PUD w/
> PAGE_SIZE=4K, leading to 513 pages as proposed in the series. Exposing
> that configuration option in a module parameter is presumptive that all
> VMs on a host use the exact same memory configuration, which doesn't
> feel right to me.

Can you elaborate on the cache size needing to be tied to the backing source?
Do the issues arise if you get to a point where KVM can have PGD-sized hugepages
with PAGE_SIZE=4KiB?  Or do you want to let userspace optimize _now_ for PMD+4KiB?