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Tue, 16 Apr 2024 11:07:14 -0700 (PDT) Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 References: <20240416095638.3620345-1-tabba@google.com> <20240416095638.3620345-10-tabba@google.com> In-Reply-To: From: Fuad Tabba Date: Tue, 16 Apr 2024 19:06:37 +0100 Message-ID: Subject: Re: [PATCH v2 09/47] KVM: arm64: Avoid BBM when changing only s/w bits in Stage-2 PTE To: Oliver Upton Cc: kvmarm@lists.linux.dev, maz@kernel.org, will@kernel.org, qperret@google.com, seanjc@google.com, alexandru.elisei@arm.com, catalin.marinas@arm.com, philmd@linaro.org, james.morse@arm.com, suzuki.poulose@arm.com, mark.rutland@arm.com, broonie@kernel.org, joey.gouly@arm.com, rananta@google.com, smostafa@google.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable Hi Oliver, On Tue, Apr 16, 2024 at 6:00=E2=80=AFPM Oliver Upton wrote: > > Hi Fuad, > > On Tue, Apr 16, 2024 at 10:56:00AM +0100, Fuad Tabba wrote: > > From: Will Deacon > > > > Break-before-make (BBM) can be expensive, as transitioning via an > > invalid mapping (i.e. the "break" step) requires the completion of TLB > > invalidation and can also cause other agents to fault concurrently on > > the invalid mapping. > > > > Since BBM is not required when changing only the software bits of a PTE= , > > avoid the sequence in this case and just update the PTE directly. > > > > Signed-off-by: Will Deacon > > Signed-off-by: Fuad Tabba > > --- > > arch/arm64/kvm/hyp/pgtable.c | 15 +++++++++++++++ > > 1 file changed, 15 insertions(+) > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.= c > > index 67647b853c9b..30a9a503c477 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -979,6 +979,21 @@ static int stage2_map_walker_try_leaf(const struct= kvm_pgtable_visit_ctx *ctx, > > if (!stage2_pte_needs_update(ctx->old, new)) > > return -EAGAIN; > > > > + /* If we're only changing software bits, then store them and go! = */ > > + if (!kvm_pgtable_walk_shared(ctx) && > > + !((ctx->old ^ new) & ~KVM_PTE_LEAF_ATTR_HI_SW)) { > > + bool old_is_counted =3D stage2_pte_is_counted(ctx->old); > > + > > + if (old_is_counted !=3D stage2_pte_is_counted(new)) { > > + if (old_is_counted) > > + mm_ops->put_page(ctx->ptep); > > + else > > + mm_ops->get_page(ctx->ptep); > > + } > > + WRITE_ONCE(*ctx->ptep, new); > > IIUC, you're using a naked WRITE_ONCE() to avoid a error path that > cleans up the reference count. That's fine, but can you change this to a > WARN_ON_ONCE(stage2_try_set_pte())? Should this ever be changed to > become a shared walk w/o gracefully handling a cleanup here I'd hope the > warning would catch someone's attention. I think you mean WARN_ON_ONCE(!stage2_try_set_pte(...)). I can do that, but for now, warnings in protected mode are fatal. Should I do that anyway? Cheers, /fuad > -- > Thanks, > Oliver