From: David Matlack <dmatlack@google.com>
To: Oliver Upton <oliver.upton@linux.dev>
Cc: Sean Christopherson <seanjc@google.com>,
Ricardo Koller <ricarkol@google.com>,
Marc Zyngier <maz@kernel.org>,
pbonzini@redhat.com, yuzenghui@huawei.com, kvm@vger.kernel.org,
kvmarm@lists.linux.dev, qperret@google.com,
catalin.marinas@arm.com, andrew.jones@linux.dev,
alexandru.elisei@arm.com, suzuki.poulose@arm.com,
eric.auger@redhat.com, gshan@redhat.com, reijiw@google.com,
rananta@google.com, bgardon@google.com, ricarkol@gmail.com
Subject: Re: [PATCH 6/9] KVM: arm64: Split huge pages when dirty logging is enabled
Date: Tue, 31 Jan 2023 10:01:45 -0800 [thread overview]
Message-ID: <CALzav=esLqUWd-1z=X+qzSxQDLS3Lh_cx4MAznp+rC9f-mrY0A@mail.gmail.com> (raw)
In-Reply-To: <Y9lTz3ryasgkfhs/@google.com>
On Tue, Jan 31, 2023 at 9:46 AM Oliver Upton <oliver.upton@linux.dev> 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.
I don't see how this can be true if we are able to tolerate splitting
2M pages. Splitting 2M pages inherently means 512 Break-Before-Make
operations per 1GiB region of guest physical memory.
If we had a cache size of 1 and were splitting a 1GiB region, we would
then need to do 512+1 BBM per 1GiB region. If we can tolerate 512 per
1GiB, why not 513?
It seems more like the 513 cache size is more to optimize splitting
1GiB pages. I agree it can turn those 513 int 1, but future versions
of the architecture also elide BBM requirements which is another way
to optimize 1GiB pages.
> 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.
>
> --
> Thanks,
> Oliver
next prev parent reply other threads:[~2023-01-31 18:02 UTC|newest]
Thread overview: 48+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-13 3:49 [PATCH 0/9] KVM: arm64: Eager Huge-page splitting for dirty-logging Ricardo Koller
2023-01-13 3:49 ` [PATCH 1/9] KVM: arm64: Add KVM_PGTABLE_WALK_REMOVED into ctx->flags Ricardo Koller
2023-01-24 0:51 ` Ben Gardon
2023-01-24 0:56 ` Oliver Upton
2023-01-24 16:32 ` Ricardo Koller
2023-01-24 18:00 ` Ben Gardon
2023-01-26 18:48 ` Ricardo Koller
2023-01-24 16:30 ` Ricardo Koller
2023-01-13 3:49 ` [PATCH 2/9] KVM: arm64: Add helper for creating removed stage2 subtrees Ricardo Koller
2023-01-14 17:58 ` kernel test robot
2023-01-24 0:55 ` Ben Gardon
2023-01-24 16:35 ` Ricardo Koller
2023-01-24 17:07 ` Oliver Upton
2023-01-13 3:49 ` [PATCH 3/9] KVM: arm64: Add kvm_pgtable_stage2_split() Ricardo Koller
2023-01-24 1:03 ` Ben Gardon
2023-01-24 16:46 ` Ricardo Koller
2023-01-24 17:11 ` Oliver Upton
2023-01-24 17:18 ` Ricardo Koller
2023-01-24 17:48 ` David Matlack
2023-01-24 20:28 ` Oliver Upton
2023-02-06 9:20 ` Zheng Chuan
2023-02-06 16:28 ` Ricardo Koller
2023-01-13 3:49 ` [PATCH 4/9] KVM: arm64: Refactor kvm_arch_commit_memory_region() Ricardo Koller
2023-01-13 3:49 ` [PATCH 5/9] KVM: arm64: Add kvm_uninit_stage2_mmu() Ricardo Koller
2023-01-13 3:49 ` [PATCH 6/9] KVM: arm64: Split huge pages when dirty logging is enabled Ricardo Koller
2023-01-24 17:52 ` Ben Gardon
2023-01-24 22:19 ` Oliver Upton
2023-01-24 22:45 ` Oliver Upton
2023-01-26 18:45 ` Ricardo Koller
2023-01-26 19:25 ` Ricardo Koller
2023-01-26 20:10 ` Marc Zyngier
2023-01-27 15:45 ` Ricardo Koller
2023-01-30 21:18 ` Oliver Upton
2023-01-31 1:18 ` Sean Christopherson
2023-01-31 17:45 ` Oliver Upton
2023-01-31 17:54 ` Sean Christopherson
2023-01-31 19:06 ` Oliver Upton
2023-01-31 18:01 ` David Matlack [this message]
2023-01-31 18:19 ` Ricardo Koller
2023-01-31 18:35 ` Oliver Upton
2023-01-31 10:31 ` Marc Zyngier
2023-01-31 10:28 ` Marc Zyngier
2023-02-06 16:35 ` Ricardo Koller
2023-01-13 3:49 ` [PATCH 7/9] KVM: arm64: Open-code kvm_mmu_write_protect_pt_masked() Ricardo Koller
2023-01-13 3:49 ` [PATCH 8/9] KVM: arm64: Split huge pages during KVM_CLEAR_DIRTY_LOG Ricardo Koller
2023-01-13 3:50 ` [PATCH 9/9] KVM: arm64: Use local TLBI on permission relaxation Ricardo Koller
2023-01-24 0:48 ` [PATCH 0/9] KVM: arm64: Eager Huge-page splitting for dirty-logging Ben Gardon
2023-01-24 16:50 ` Ricardo Koller
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