From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-29.mta1.migadu.com (out-29.mta1.migadu.com [95.215.58.29]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1E43EC942 for ; Tue, 4 Apr 2023 19:19:36 +0000 (UTC) Date: Tue, 4 Apr 2023 19:19:28 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1680635974; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bE39oER6mn2FeknwM80LYwyihVXXMbmc7AYdxCBaNKA=; b=o2lpR3z3PI8EYPXhhSNQwIzQ2F9+bFAurHLQZJzbXBEVzJFyZXWqhNt/bd1MfOXfBjuzQI P0AX3UCRyzYkstzP+ET4V+we8YsuEbIBsKbzIcit5BA73KKiBALMyWal2F4R44fMblXhJh 4e93hK5GgLcKc6db51KQ8hGeCbJp7fk= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Raghavendra Rao Ananta , h@linux.dev Cc: Oliver Upton , Marc Zyngier , Ricardo Koller , Reiji Watanabe , James Morse , Alexandru Elisei , Suzuki K Poulose , Will Deacon , Paolo Bonzini , Catalin Marinas , Jing Zhang , Colton Lewis , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2 7/7] KVM: arm64: Create a fast stage-2 unmap path Message-ID: References: <20230206172340.2639971-1-rananta@google.com> <20230206172340.2639971-8-rananta@google.com> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT On Tue, Apr 04, 2023 at 10:52:01AM -0700, Raghavendra Rao Ananta wrote: > On Wed, Mar 29, 2023 at 5:42 PM Oliver Upton wrote: > > > > On Mon, Feb 06, 2023 at 05:23:40PM +0000, Raghavendra Rao Ananta wrote: > > > The current implementation of the stage-2 unmap walker > > > traverses the entire page-table to clear and flush the TLBs > > > for each entry. This could be very expensive, especially if > > > the VM is not backed by hugepages. The unmap operation could be > > > made efficient by disconnecting the table at the very > > > top (level at which the largest block mapping can be hosted) > > > and do the rest of the unmapping using free_removed_table(). > > > If the system supports FEAT_TLBIRANGE, flush the entire range > > > that has been disconnected from the rest of the page-table. > > > > > > Suggested-by: Ricardo Koller > > > Signed-off-by: Raghavendra Rao Ananta > > > --- > > > arch/arm64/kvm/hyp/pgtable.c | 44 ++++++++++++++++++++++++++++++++++++ > > > 1 file changed, 44 insertions(+) > > > > > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c > > > index 0858d1fa85d6b..af3729d0971f2 100644 > > > --- a/arch/arm64/kvm/hyp/pgtable.c > > > +++ b/arch/arm64/kvm/hyp/pgtable.c > > > @@ -1017,6 +1017,49 @@ static int stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx, > > > return 0; > > > } > > > > > > +/* > > > + * The fast walker executes only if the unmap size is exactly equal to the > > > + * largest block mapping supported (i.e. at KVM_PGTABLE_MIN_BLOCK_LEVEL), > > > + * such that the underneath hierarchy at KVM_PGTABLE_MIN_BLOCK_LEVEL can > > > + * be disconnected from the rest of the page-table without the need to > > > + * traverse all the PTEs, at all the levels, and unmap each and every one > > > + * of them. The disconnected table is freed using free_removed_table(). > > > + */ > > > +static int fast_stage2_unmap_walker(const struct kvm_pgtable_visit_ctx *ctx, > > > + enum kvm_pgtable_walk_flags visit) > > > +{ > > > + struct kvm_pgtable_mm_ops *mm_ops = ctx->mm_ops; > > > + kvm_pte_t *childp = kvm_pte_follow(ctx->old, mm_ops); > > > + struct kvm_s2_mmu *mmu = ctx->arg; > > > + > > > + if (!kvm_pte_valid(ctx->old) || ctx->level != KVM_PGTABLE_MIN_BLOCK_LEVEL) > > > + return 0; > > > + > > > + if (!stage2_try_break_pte(ctx, mmu)) > > > + return -EAGAIN; > > > + > > > + /* > > > + * Gain back a reference for stage2_unmap_walker() to free > > > + * this table entry from KVM_PGTABLE_MIN_BLOCK_LEVEL - 1. > > > + */ > > > + mm_ops->get_page(ctx->ptep); > > > > Doesn't this run the risk of a potential UAF if the refcount was 1 before > > calling stage2_try_break_pte()? IOW, stage2_try_break_pte() will drop > > the refcount to 0 on the page before this ever gets called. > > > > Also, AFAICT this misses the CMOs that are required on systems w/o > > FEAT_FWB. Without them it is possible that the host will read something > > other than what was most recently written by the guest if it is using > > noncacheable memory attributes at stage-1. > > > > I imagine the actual bottleneck is the DSB required after every > > CMO/TLBI. Theoretically, the unmap path could be updated to: > > > > - Perform the appropriate CMOs for every valid leaf entry *without* > > issuing a DSB. > > > > - Elide TLBIs entirely that take place in the middle of the walk > > > > - After the walk completes, dsb(ish) to guarantee that the CMOs have > > completed and the invalid PTEs are made visible to the hardware > > walkers. This should be done implicitly by the TLBI implementation > > > > - Invalidate the [addr, addr + size) range of IPAs > > > > This would also avoid over-invalidating stage-1 since we blast the > > entire stage-1 context for every stage-2 invalidation. Thoughts? > > > Correct me if I'm wrong, but if we invalidate the TLB after the walk > is complete, don't you think there's a risk of race if the guest can > hit in the TLB even though the page was unmapped? Yeah, we'd need to do the CMOs _after_ making the translation invalid in the page tables and completing the TLB invalidation. Apologies. Otherwise, the only requirement we need to uphold w/ either the MMU notifiers or userspace is that the translation has been invalidated at the time of return. -- Thanks, Oliver From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id A6F1DC761A6 for ; Tue, 4 Apr 2023 19:20:42 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:Cc:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=7kxEmmBTD51bKbIr6DzvSYcm/GcnmU53BAJBTI/jQ00=; b=3i384d9pITam8c NmW9xOFtseUJmgutbb6fMS0PxZgHTlUQPOSHOj2NEd8BYz6HTUf5gQxok68c/eYNLkOlOtL1+t77B P1jNFwzmMyaovhKKIkjOPi5zXOlJtIhDFm6HKz2iaVh319d6oAM8ftjwjjTCQpxX+uhyVmGP27Y1j bdk6+mjKfi8uUo9lDqupAnW/afuHjyhwXFaF0AdpXZE1aCmT+Gpyj4Dgr0G1ifzrOsHCof9141gM7 Ou9rYGatFh8yrL00Sc4OeWMVC6wkg13dxLOBuG6P9mvHR83xQZS8YleLrLdahqsAM/KfLtfqUlw5G WGDgmM2Yb5Zp7r/26PJQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.96 #2 (Red Hat Linux)) id 1pjmBy-002brh-0Q; Tue, 04 Apr 2023 19:19:42 +0000 Received: from out-57.mta1.migadu.com ([2001:41d0:203:375::39]) by bombadil.infradead.org with esmtps (Exim 4.96 #2 (Red Hat Linux)) id 1pjmBu-002bq7-0M for linux-arm-kernel@lists.infradead.org; Tue, 04 Apr 2023 19:19:40 +0000 Date: Tue, 4 Apr 2023 19:19:28 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1680635974; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=bE39oER6mn2FeknwM80LYwyihVXXMbmc7AYdxCBaNKA=; b=o2lpR3z3PI8EYPXhhSNQwIzQ2F9+bFAurHLQZJzbXBEVzJFyZXWqhNt/bd1MfOXfBjuzQI P0AX3UCRyzYkstzP+ET4V+we8YsuEbIBsKbzIcit5BA73KKiBALMyWal2F4R44fMblXhJh 4e93hK5GgLcKc6db51KQ8hGeCbJp7fk= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Raghavendra Rao Ananta , h@linux.dev Cc: Oliver Upton , Marc Zyngier , Ricardo Koller , Reiji Watanabe , James Morse , Alexandru Elisei , Suzuki K Poulose , Will Deacon , Paolo Bonzini , Catalin Marinas , Jing Zhang , Colton Lewis , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v2 7/7] KVM: arm64: Create a fast stage-2 unmap path Message-ID: References: <20230206172340.2639971-1-rananta@google.com> <20230206172340.2639971-8-rananta@google.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20230404_121938_718739_9F313B31 X-CRM114-Status: GOOD ( 39.82 ) 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: 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