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Mon, 9 Nov 2026 05:55:14 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1794232516; bh=KJJLhpniQZUmQ77hpm1zsDXOiW1CCXxKt62dmhbtma8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=b8Usqek3kaVUOB1RB0xwIA04Lp+O+RMitLE1+TFQAq4X64T9fjtId114taMH3frRl uKaZaRYI3VzAPYg9BkjXhmSjl9Z9fCVabsIalNzEHG3yDozSmuQbmFGDj9pvCUCHl0 RU9u7X30AsjjJzRT1Z8wrFgLwHe0DDdE1gdE9jI4= From: Leonardo Bras To: Suzuki K Poulose Cc: Leonardo Bras , Marc Zyngier , Oliver Upton , Fuad Tabba , Joey Gouly , Steffen Eiden , Zenghui Yu , Catalin Marinas , Will Deacon , Mark Rutland , Raghavendra Rao Ananta , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org Subject: Re: [PATCH RESEND v4 2/3] KVM: arm64: Introduce KVM_PGTABLE_WALK_SKIP_LEVEL* walk flags Date: Mon, 9 Nov 2026 13:55:05 +0000 Message-ID: X-Mailer: git-send-email 2.56.0 In-Reply-To: <60fe23f4-4fca-41b6-8489-a28ab7b281f9@arm.com> References: <20261106161240.1345126-1-leo.bras@arm.com> <20261106161240.1345126-3-leo.bras@arm.com> <60fe23f4-4fca-41b6-8489-a28ab7b281f9@arm.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8bit On Fri, Nov 06, 2026 at 05:02:31PM +0000, Suzuki K Poulose wrote: > On 06/11/2026 16:12, Leonardo Bras wrote: > > Add new walking flags KVM_PGTABLE_WALK_SKIP_LEVEL{0-3}. > > When used, they make the kvm_pgtable_walk() skip the given level, as well > > as levels higher than that. > > > > Being able to skip levels substancially decreases the runtime of a > > minor nit: substantially fixed! > > > pagetable walk when the callback function does nothing on that level. > > > > Signed-off-by: Leonardo Bras > > --- > > arch/arm64/include/asm/kvm_pgtable.h | 14 ++++++++++++++ > > arch/arm64/kvm/hyp/pgtable.c | 16 +++++++++++++--- > > 2 files changed, 27 insertions(+), 3 deletions(-) > > > > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h > > index 41a8687938eb..5f37ceef8d63 100644 > > --- a/arch/arm64/include/asm/kvm_pgtable.h > > +++ b/arch/arm64/include/asm/kvm_pgtable.h > > @@ -311,31 +311,45 @@ typedef bool (*kvm_pgtable_force_pte_cb_t)(u64 addr, u64 end, > > * @KVM_PGTABLE_WALK_SHARED: Indicates the page-tables may be shared > > * with other software walkers. > > * @KVM_PGTABLE_WALK_IGNORE_EAGAIN: Don't terminate the walk early if > > * the walker returns -EAGAIN. > > * @KVM_PGTABLE_WALK_SKIP_BBM_TLBI: Visit and update table entries > > * without Break-before-make's > > * TLB invalidation. > > * @KVM_PGTABLE_WALK_SKIP_CMO: Visit and update table entries > > * without Cache maintenance > > * operations required. > > + * @KVM_PGTABLE_WALK_SKIP_LEVEL0: Skip visiting level 0+ entries > > + * @KVM_PGTABLE_WALK_SKIP_LEVEL1: Skip visiting level 1+ entries > > + * @KVM_PGTABLE_WALK_SKIP_LEVEL2: Skip visiting level 2+ entries > > + * @KVM_PGTABLE_WALK_SKIP_LEVEL3: Skip visiting level 3 entries > > */ > > enum kvm_pgtable_walk_flags { > > KVM_PGTABLE_WALK_LEAF = BIT(0), > > KVM_PGTABLE_WALK_TABLE_PRE = BIT(1), > > KVM_PGTABLE_WALK_TABLE_POST = BIT(2), > > KVM_PGTABLE_WALK_SHARED = BIT(3), > > KVM_PGTABLE_WALK_IGNORE_EAGAIN = BIT(4), > > KVM_PGTABLE_WALK_SKIP_BBM_TLBI = BIT(5), > > KVM_PGTABLE_WALK_SKIP_CMO = BIT(6), > > + /* SKIP_LEVEL* flags should not be split nor reordered */ > > + KVM_PGTABLE_WALK_SKIP_LEVEL0 = BIT(7), > > + KVM_PGTABLE_WALK_SKIP_LEVEL1 = BIT(8), > > + KVM_PGTABLE_WALK_SKIP_LEVEL2 = BIT(9), > > + KVM_PGTABLE_WALK_SKIP_LEVEL3 = BIT(10), > > }; > > +#define KVM_PGTABLE_WALK_SKIP_LEVELS (KVM_PGTABLE_WALK_SKIP_LEVEL0 | \ > > + KVM_PGTABLE_WALK_SKIP_LEVEL1 | \ > > + KVM_PGTABLE_WALK_SKIP_LEVEL2 | \ > > + KVM_PGTABLE_WALK_SKIP_LEVEL3 ) > > + > > struct kvm_pgtable_visit_ctx { > > kvm_pte_t *ptep; > > kvm_pte_t old; > > void *arg; > > struct kvm_pgtable_mm_ops *mm_ops; > > u64 start; > > u64 addr; > > u64 end; > > s8 level; > > enum kvm_pgtable_walk_flags flags; > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > index 7a51f78b6446..80744478c810 100644 > > --- a/arch/arm64/kvm/hyp/pgtable.c > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > @@ -137,20 +137,27 @@ static bool kvm_pgtable_walk_continue(const struct kvm_pgtable_walker *walker, > > * Ignore the return code altogether for walkers outside a fault handler > > * (e.g. write protecting a range of memory) and chug along with the > > * page table walk. > > */ > > if (r == -EAGAIN) > > return walker->flags & KVM_PGTABLE_WALK_IGNORE_EAGAIN; > > return !r; > > } > > +static bool kvm_pgtable_skip_level(s8 level, enum kvm_pgtable_walk_flags flags) > > +{ > > + u32 skip = FIELD_GET(KVM_PGTABLE_WALK_SKIP_LEVELS, flags); > > + > > + return unlikely(skip && level >= ffs(skip) - 1); > > +} > > + > > static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data, > > struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level); > > static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data, > > struct kvm_pgtable_mm_ops *mm_ops, > > kvm_pteref_t pteref, s8 level) > > { > > enum kvm_pgtable_walk_flags flags = data->walker->flags; > > kvm_pte_t *ptep = kvm_dereference_pteref(data->walker, pteref); > > struct kvm_pgtable_visit_ctx ctx = { > > @@ -185,35 +192,35 @@ static inline int __kvm_pgtable_visit(struct kvm_pgtable_walk_data *data, > > * into a newly installed or replaced table. > > */ > > if (reload) { > > ctx.old = READ_ONCE(*ptep); > > table = kvm_pte_table(ctx.old, level); > > } > > if (!kvm_pgtable_walk_continue(data->walker, ret)) > > return ret; > > - if (!table) { > > + if (!table || kvm_pgtable_skip_level(level + 1, ctx.flags)) { > > data->addr = ALIGN_DOWN(data->addr, kvm_granule_size(level)); > > data->addr += kvm_granule_size(level); > > goto out; > > } > > childp = (kvm_pteref_t)kvm_pte_follow(ctx.old, mm_ops); > > ret = __kvm_pgtable_walk(data, mm_ops, childp, level + 1); > > if (!kvm_pgtable_walk_continue(data->walker, ret)) > > return ret; > > - if (ctx.flags & KVM_PGTABLE_WALK_TABLE_POST) > > +out: > > + if (table && ctx.flags & KVM_PGTABLE_WALK_TABLE_POST) > > ret = kvm_pgtable_visitor_cb(data, &ctx, KVM_PGTABLE_WALK_TABLE_POST); > > -out: > > if (kvm_pgtable_walk_continue(data->walker, ret)) > > return 0; > > return ret; > > } > > static int __kvm_pgtable_walk(struct kvm_pgtable_walk_data *data, > > struct kvm_pgtable_mm_ops *mm_ops, kvm_pteref_t pgtable, s8 level) > > { > > u32 idx; > > @@ -242,20 +249,23 @@ static int _kvm_pgtable_walk(struct kvm_pgtable *pgt, struct kvm_pgtable_walk_da > > u32 idx; > > int ret = 0; > > u64 limit = BIT(pgt->ia_bits); > > if (data->addr > limit || data->end > limit) > > return -ERANGE; > > if (!pgt->pgd) > > return -EINVAL; > > + if (kvm_pgtable_skip_level(pgt->start_level, data->walker->flags)) > > + return 0; > > + > > Should we rather return an error here ? A walk skipping the start level, > is that any meaningful ? > My rationale on not returning error is that it may be ok for some routine to call a walker on a pagetable range just to be sure, even if it actually does nothing. I take that this happens based on the split walker being called on kvm_arch_mmu_enable_log_dirty_pt_masked() multiple times in the same range, even though it's already split, for instance. But I am fine on changing it to return an error here if it's important in some scenario. Thanks! Leo