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From: Oliver Upton To: Marc Zyngier Cc: Colton Lewis , kvm@vger.kernel.org, Catalin Marinas , Will Deacon , James Morse , Suzuki K Poulose , Zenghui Yu , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev Subject: Re: [PATCH 3/3] KVM: arm64: Skip break phase when we have FEAT_BBM level 2 Message-ID: References: <20230602170147.1541355-1-coltonlewis@google.com> <20230602170147.1541355-4-coltonlewis@google.com> <87sfb7octw.wl-maz@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <87sfb7octw.wl-maz@kernel.org> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230605_143615_674045_187EDDF3 X-CRM114-Status: GOOD ( 22.56 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sun, Jun 04, 2023 at 09:23:39AM +0100, Marc Zyngier wrote: > On Fri, 02 Jun 2023 18:01:47 +0100, Colton Lewis wrote: > > +static bool stage2_try_make_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu, kvm_pte_t new) > > { > > struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops; > > > > - WARN_ON(!stage2_pte_is_locked(*ctx->ptep)); > > + if (!stage2_has_bbm_level2()) > > + WARN_ON(!stage2_pte_is_locked(*ctx->ptep)); > > + > > + if (!stage2_try_set_pte(ctx, new)) > > + return false; > > + > > + if (kvm_pte_table(ctx->old, ctx->level)) > > + kvm_call_hyp(__kvm_tlb_flush_vmid, mmu); > > + else if (kvm_pte_valid(ctx->old) && !stage2_pte_perms_equal(ctx->old, new)) > > + kvm_call_hyp(__kvm_tlb_flush_vmid_ipa_nsh, mmu, ctx->addr, ctx->level); > > Why a non-shareable invalidation? Nothing in this code captures the > rationale for it. What if the permission change was a *restriction* of > the permission? It should absolutely be global, and not local. IIRC, Colton was testing largely with permission relaxation, and had forward progress issues b.c. the stale TLB entry was never invalidated in response to a permission fault. Nonetheless, I very much agree with your suggestion. Non-Shareable invalidations should only be applied after exhausting all other invalidation requirements for a particular manipulation to the stage-2 tables. > > > > if (stage2_pte_is_counted(new)) > > mm_ops->get_page(ctx->ptep); > > > > - smp_store_release(ctx->ptep, new); > > + return true; > > } > > > > static void stage2_put_pte(const struct kvm_pgtable_visit_ctx *ctx, struct kvm_s2_mmu *mmu, > > @@ -879,7 +917,8 @@ static int stage2_map_walker_try_leaf(const struct kvm_pgtable_visit_ctx *ctx, > > stage2_pte_executable(new)) > > mm_ops->icache_inval_pou(kvm_pte_follow(new, mm_ops), granule); > > > > - stage2_make_pte(ctx, new); > > + if (!stage2_try_make_pte(ctx, data->mmu, new)) > > + return -EAGAIN; > > So we don't have forward-progress guarantees anymore? I'm not sure > this is a change I'm overly fond of. I'll take the blame for the clunky wording here, though I do not believe there are any real changes to our forward progress guarantees relative to the existing code. Previously, we did the CAS on the break side of things to have a fault handler 'take ownership' of a PTE. The CAS now needs to move onto the make end when doing a BBM=2 style manipulation. Would you rather see something explicitly keyed on the BBM capability here? Then we could use a helper that implies unconditional success for BBM!=2 systems. -- Thanks, Oliver _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel