From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 EB3B41448E5 for ; Mon, 8 Apr 2024 15:53:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712591613; cv=none; b=WoLiJTPtRTmW+T8NV/UZHW8E8QqxBxX3CNaZ8XUJNfMBuDID9RkMEZO0V4NyevothwEy059zSDgoYm+zle4svz8HCd+kAQjXF9jfcs7LGoYe660XxsBhCHKugEZevIOjlXCLPSGnQS5Mv6vKelvMzNhOGQUxr5MJScbuKLdj3uk= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1712591613; c=relaxed/simple; bh=kT5MyaZOJUuTHHNiQ3YwyX5aHe0RbhSguBEaXAN1oVo=; h=Date:Message-ID:From:To:Cc:Subject:In-Reply-To:References: MIME-Version:Content-Type; b=aEsFivxtN4y2dJTDkzN98ZAmTHmIoyvlhRP4S4iXMSj0yGVlmIsVCT48+6AwUHIfvFntlAyhNqWvoVinGWnx0g2zMAZLYSVHVSVhuclIL3YwWFe8VnkeXpVDS0EXo2RKcSNlBNpJyAL7PVbpsj3vgc11QZzVkJyhpzul74W39yM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=C1Qs9utu; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="C1Qs9utu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 77272C433C7; Mon, 8 Apr 2024 15:53:32 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1712591612; bh=kT5MyaZOJUuTHHNiQ3YwyX5aHe0RbhSguBEaXAN1oVo=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=C1Qs9utuJKWF64gvod4lUQeTC9sh3mdqc5c3tjGZWEzFm18h23PWm4YwThKsY+G+w pr7E4uBq7kSHfT0uqvddPORIZPwFFCZojEGYc+phLoFYROE/RvBI9gdtqvV0aRwn3S qLYX9hzpjmOckU4z8ty9EpwXtOBj7O5irfo4Pfym0m33ASmBydn2l4b349dd4huDju q3S7iYg5orNQA666KXBXsALXyOObGvzSPgPq6P8k+vFQ8Y6l0TKATFylVPsTKuuZaC I770EEF2HvCJFysbtuIWzqsaDgu6/2q1CtiXYQsjkHNWDTUU9Hm7MU4DsT1NSZd970 eyooOYJPfH51g== Received: from sofa.misterjones.org ([185.219.108.64] helo=goblin-girl.misterjones.org) by disco-boy.misterjones.org with esmtpsa (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.95) (envelope-from ) id 1rtrJJ-002X34-Os; Mon, 08 Apr 2024 16:53:29 +0100 Date: Mon, 08 Apr 2024 16:53:29 +0100 Message-ID: <86zfu3sveu.wl-maz@kernel.org> From: Marc Zyngier To: Fuad Tabba Cc: kvmarm@lists.linux.dev, 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, oliver.upton@linux.dev, mark.rutland@arm.com, broonie@kernel.org, joey.gouly@arm.com, rananta@google.com Subject: Re: [PATCH v1 03/44] KVM: arm64: Move setting the page as dirty out of the critical section In-Reply-To: References: <20240327173531.1379685-1-tabba@google.com> <20240327173531.1379685-4-tabba@google.com> <864jccti69.wl-maz@kernel.org> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/29.2 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: tabba@google.com, kvmarm@lists.linux.dev, 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, oliver.upton@linux.dev, mark.rutland@arm.com, broonie@kernel.org, joey.gouly@arm.com, rananta@google.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Mon, 08 Apr 2024 16:41:15 +0100, Fuad Tabba wrote: >=20 > Hi Marc, >=20 > On Mon, Apr 8, 2024 at 8:41=E2=80=AFAM Marc Zyngier wrot= e: > > > > On Wed, 27 Mar 2024 17:34:50 +0000, > > Fuad Tabba wrote: > > > > > > Move the unlock earlier in user_mem_abort() to shorten the > > > critical section. This also helps for future refactoring and > > > reuse of similar code. > > > > > > Signed-off-by: Fuad Tabba > > > --- > > > arch/arm64/kvm/mmu.c | 8 +++++--- > > > 1 file changed, 5 insertions(+), 3 deletions(-) > > > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > > index 18680771cdb0..3afc42d8833e 100644 > > > --- a/arch/arm64/kvm/mmu.c > > > +++ b/arch/arm64/kvm/mmu.c > > > @@ -1522,8 +1522,10 @@ static int user_mem_abort(struct kvm_vcpu *vcp= u, phys_addr_t fault_ipa, > > > > > > read_lock(&kvm->mmu_lock); > > > pgt =3D vcpu->arch.hw_mmu->pgt; > > > - if (mmu_invalidate_retry(kvm, mmu_seq)) > > > + if (mmu_invalidate_retry(kvm, mmu_seq)) { > > > + ret =3D -EAGAIN; > > > goto out_unlock; > > > + } > > > > > > /* > > > * If we are not forced to use page mapping, check if we are > > > @@ -1581,6 +1583,8 @@ static int user_mem_abort(struct kvm_vcpu *vcpu= , phys_addr_t fault_ipa, > > > memcache, > > > KVM_PGTABLE_WALK_HANDLE_FA= ULT | > > > KVM_PGTABLE_WALK_SHARED); > > > +out_unlock: > > > + read_unlock(&kvm->mmu_lock); > > > > > > /* Mark the page dirty only if the fault is handled successfull= y */ > > > if (writable && !ret) { > > > @@ -1588,8 +1592,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu= , phys_addr_t fault_ipa, > > > mark_page_dirty_in_slot(kvm, memslot, gfn); > > > } > > > > > > -out_unlock: > > > - read_unlock(&kvm->mmu_lock); > > > kvm_release_pfn_clean(pfn); > > > return ret !=3D -EAGAIN ? ret : 0; > > > } > > > > It now means that things such as marking a page dirty happens outside > > of the lock, which may interact with the dirty log/bitmap stuff. > > > > Can you elaborate on *why* this is correct? >=20 > The reason why I _think_ this is correct (something I am less certain > about now judging by your reply :) is that this is a lock that > protects the stage-2 page tables (struct kvm_vcpu_arch::hw_mmu), held > in this case for read. As far as I can tell, kvm_set_pfn_dirty() and > mark_page_dirty_in_slot() only access and modify (i.e., write to) the > page without accessing the stage-2 page table. I think that the dirtly > log is protected by the slots_lock. >=20 > Am I missing something? I'm just not sure, nothing more sinister than that. I was also reviewing 20240402213656.3068504-1-dmatlack@google.com, which also moves the access to the dirty bitmap outside of the critical section. I haven't had a chance to page the whole thing in, unfortunately, hence my question. In any case, it would be work documenting the rationale for this relaxation. Thanks, M. --=20 Without deviation from the norm, progress is not possible.