From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-wr1-f49.google.com (mail-wr1-f49.google.com [209.85.221.49]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CF78222D4E2 for ; Sat, 19 Jul 2025 07:58:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.49 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752911907; cv=none; b=EUdZQ2n3Y+620eLqH+xN8RtFJ3ih6WL7V7bXCqY+/8VORv1cHEuCZd/D/pFAKy8O51JZPkxOYXrOOdXjdonki+f4TNo92exUZil7uiBQDhLWE/NCt9kqrMXjHc0sDvwMBjNCp5+T1c4n5Aw1Fg2oDOAi2sy75yMzLRJHHo+3od8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752911907; c=relaxed/simple; bh=tJ1uWbUjnxLlGM3QoOhkMA4nYAs8lfUm3ulVKnjtvSU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=In1zsrOL6Ok8U8Y0cfxTrrtIHvh9c99sX40qDJS5hryEV3fk9TkIQ82+ICRlYgK1p6zCY03DlWd7m0o1bnUhxiaWKkHYKt5LwmkZXiv232Zs/fae90JdtE3XGJPQ3JXSgnKsBHzPJV3WZSZ7MJiUePzdZiU+2J3YFaq2HEsxaJs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=vODRF+kC; arc=none smtp.client-ip=209.85.221.49 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="vODRF+kC" Received: by mail-wr1-f49.google.com with SMTP id ffacd0b85a97d-3a548a73ff2so2567341f8f.0 for ; Sat, 19 Jul 2025 00:58:25 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1752911904; x=1753516704; darn=vger.kernel.org; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:from:to:cc:subject:date:message-id:reply-to; bh=TfRByUZViUlkhtvwbgJ9kqHK2zRV37bvsO8dM/mnyeg=; b=vODRF+kC95FT79zjGsl+L9B3K1IsbTrfSFnd7wsYCj/GmlykdgMt4NaRn3GYHZtrRr pskjfi6iMiM4SZKN6xu7/4E4L8Mf08++LLxiZv3dMYrY+GGNClqi4kfp7VbKGKLyK+9F NMNR73ak2Ui35J5ouqolASqhz6CQxK2lAyzkYKq25vdNSO6AZASgG8/0OZ3XYzDoIjV9 GOaPdMvOhsalcO17ySnZXpwAbtNDCzR8T+Ezw+Yry+42pjftTAdM331+GXnBfjVgFqXB NGQyxWtAbgmnt18+m/VR4Vd4dePGXURR9gtCBF2381pKASSgUWS8RrgXjmxOpWjrMwsy QsSg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1752911904; x=1753516704; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=TfRByUZViUlkhtvwbgJ9kqHK2zRV37bvsO8dM/mnyeg=; b=G5h6Op9389pW9j4fW5CiOml2wOaNIe9mk5Wo6p+D0C19SCW8j4gLGG+cb5ocJcjgzI QzA/Puqz3jfjN9VmUY090WXpvqDmqQFetzeJTRAonIMTygnd/Gd2h0wVuc82bksRpGFz HTPcqvrov0CdROF4qiq6Q8akBbTO29Xah1B+Bt8x94V19JPz8mWXFs1u4mva/Dj6Q5wN 7oFp1lyJF5CZ2yFvmJ/9tjiHHFlVvaaHkvMfhjXsmPV2JYCNotGnrNxojaWSUJqQWdLz yXQD8xKkSC5HjH83XHnn65aX6S14kzIUh//0NRWE6XkeZn1uPRt9Kvld0sVIvvlEQhaN ROWQ== X-Forwarded-Encrypted: i=1; AJvYcCXeAJeGdFBdBdx5SzRlyEJNRJI9wT0WdQsgkya1uUEqJTxUgtJvHzgmNE4sybdqzv1hiBxZk/zd++mIYnA=@vger.kernel.org X-Gm-Message-State: AOJu0YxppwDRB/2bQV8oKB4v2/uyBW1zQ0LdOg+ziLKl2+NZAl+GJatC NAl1Ex+bbVPcRK4PIvl3NwAJIjmmtktoBo+ovdnyNv3e5nJ7aSGEsATN8SNcD6XTFA== X-Gm-Gg: ASbGnctSOUPw3f6bRS2GnPMroUx4XUyH4w1vG8g+jhXNfajtDEjURUX+WTs9nMOFcnU LY5WsnCBcx+/eO6PzUHdvH8I0KcXxeDpBljcu6qmjDhOYbLmoOobW0efeYLKk1ATHg46jt8RHuk d2tjH7h2iDrOocGSOJWm69W8nm1UdpQL+oXJn1xxUl3xLbxrHoexZNp5/wnMF2txoY2Pqr/0TN3 EMGNTi63beDzh80pJliLapjFTVQxbv/aZ+8WkqcOaHtHVfaSvzYvDZ5NUlGiOKyzWeq2k/N9jqw ZxNTmb1nqsaA+YrZkPHO0ueJ5vsqJs1TuTXfXkEJUhE3nxblcel7GIigiFWSTrX8QCiWT+2ITnS CQE0UMFkOgGjUz/vvXwkHO6McXlkHu0u0D5SKjTvLrX3JAeT+kA6ArFALbxX44Lj0VfjFng== X-Google-Smtp-Source: AGHT+IH9Zj8Sie+1djBoT8jRSDy8Q+NDwYSbOAjVh0kWnwssstjjIHWaSUiNz3Ecv1vUll7ozzg2eA== X-Received: by 2002:a05:6000:42c6:b0:3a3:7115:5e7a with SMTP id ffacd0b85a97d-3b60dd7b0dbmr8359452f8f.42.1752911903972; Sat, 19 Jul 2025 00:58:23 -0700 (PDT) Received: from google.com (120.142.205.35.bc.googleusercontent.com. [35.205.142.120]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3b61ca2b803sm3923821f8f.19.2025.07.19.00.58.23 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 19 Jul 2025 00:58:23 -0700 (PDT) Date: Sat, 19 Jul 2025 07:58:19 +0000 From: Keir Fraser To: Yao Yuan Cc: Sean Christopherson , Yao Yuan , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, Eric Auger , Oliver Upton , Marc Zyngier , Will Deacon , Paolo Bonzini Subject: Re: [PATCH v2 2/4] KVM: arm64: vgic: Explicitly implement vgic_dist::ready ordering Message-ID: References: <20250716110737.2513665-1-keirf@google.com> <20250716110737.2513665-3-keirf@google.com> <5zpxxmymnyzncdnewdonnglvmvbtggjyxyqvkf6yars2bbyr4b@gottasrtoq2s> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <5zpxxmymnyzncdnewdonnglvmvbtggjyxyqvkf6yars2bbyr4b@gottasrtoq2s> On Sat, Jul 19, 2025 at 10:15:56AM +0800, Yao Yuan wrote: > On Fri, Jul 18, 2025 at 08:00:17AM -0700, Sean Christopherson wrote: > > On Thu, Jul 17, 2025, Yao Yuan wrote: > > > On Wed, Jul 16, 2025 at 11:07:35AM +0800, Keir Fraser wrote: > > > > In preparation to remove synchronize_srcu() from MMIO registration, > > > > remove the distributor's dependency on this implicit barrier by > > > > direct acquire-release synchronization on the flag write and its > > > > lock-free check. > > > > > > > > Signed-off-by: Keir Fraser > > > > --- > > > > arch/arm64/kvm/vgic/vgic-init.c | 11 ++--------- > > > > 1 file changed, 2 insertions(+), 9 deletions(-) > > > > > > > > diff --git a/arch/arm64/kvm/vgic/vgic-init.c b/arch/arm64/kvm/vgic/vgic-init.c > > > > index 502b65049703..bc83672e461b 100644 > > > > --- a/arch/arm64/kvm/vgic/vgic-init.c > > > > +++ b/arch/arm64/kvm/vgic/vgic-init.c > > > > @@ -567,7 +567,7 @@ int kvm_vgic_map_resources(struct kvm *kvm) > > > > gpa_t dist_base; > > > > int ret = 0; > > > > > > > > - if (likely(dist->ready)) > > > > + if (likely(smp_load_acquire(&dist->ready))) > > > > return 0; > > > > > > > > mutex_lock(&kvm->slots_lock); > > > > @@ -598,14 +598,7 @@ int kvm_vgic_map_resources(struct kvm *kvm) > > > > goto out_slots; > > > > } > > > > > > > > - /* > > > > - * kvm_io_bus_register_dev() guarantees all readers see the new MMIO > > > > - * registration before returning through synchronize_srcu(), which also > > > > - * implies a full memory barrier. As such, marking the distributor as > > > > - * 'ready' here is guaranteed to be ordered after all vCPUs having seen > > > > - * a completely configured distributor. > > > > - */ > > > > - dist->ready = true; > > > > + smp_store_release(&dist->ready, true); > > > > > > No need the store-release and load-acquire for replacing > > > synchronize_srcu_expedited() w/ call_srcu() IIUC: > > > > This isn't about using call_srcu(), because it's not actually about kvm->buses. > > This code is concerned with ensuring that all stores to kvm->arch.vgic are ordered > > before the store to set kvm->arch.vgic.ready, so that vCPUs never see "ready==true" > > with a half-baked distributor. > > > > In the current code, kvm_vgic_map_resources() relies on the synchronize_srcu() in > > kvm_io_bus_register_dev() to provide the ordering guarantees. Switching to > > smp_store_release() + smp_load_acquire() removes the dependency on the > > synchronize_srcu() so that the synchronize_srcu() call can be safely removed. > > Yes, I understand this and agree with your point. > > Just for discusstion: I thought it should also work even w/o > introduce the load acqure + store release after switch to > call_srcu(): The smp_mb() in call_srcu() order the all store > to kvm->arch.vgic before store kvm->arch.vgic.ready in > current implementation. The load-acquire would still be required, to ensure that accesses to kvm->arch.vgic do not get reordered earlier than the lock-free check of kvm->arch.vgic.ready. Otherwise that CPU could see that the vgic is initialised, but then use speculated reads of uninitialised vgic state. > >