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 0B9053FB0A; Wed, 6 Dec 2023 15:18:13 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="IbN6B60n" Received: by smtp.kernel.org (Postfix) with ESMTPSA id E4468C433C7; Wed, 6 Dec 2023 15:18:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1701875893; bh=j+CA2PrRtdABKk9fjTuTndRIHtJrAoLVlG643odbEJE=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=IbN6B60nkT4uB0IC9JH/LsPKbKBOvFjZxllADc63zL0mymf3IT0vRnbRgYU3iO4E1 PzXQDILI2V5uGCuSMPHFUjDBONn02bxSvju7B7Q1XOb8lm7fEQbyTKTHt1pX91tsrT THLMWsRgu3sFKO6384v5edb2RRBdLLLYE65Ts/u5KiVRDGrBR5qQ8dh6KPSva+7xt8 4WTUIaCN9W7xDbMrL/g1Wgfekc4nQJ84n5T1wI5Yb++gBs1/J4QjndKS+SULvVGTtz e0YqRxLM5Eg1GEAuF02Xc+FYiQbCTso99PkKGwyNLWkYM7ucHqIlY5kRpqb/yht+U/ q+k5TLGU7XJlg== Date: Wed, 6 Dec 2023 16:18:05 +0100 From: Lorenzo Pieralisi To: Jason Gunthorpe Cc: Catalin Marinas , Marc Zyngier , ankita@nvidia.com, Shameerali Kolothum Thodi , oliver.upton@linux.dev, suzuki.poulose@arm.com, yuzenghui@huawei.com, will@kernel.org, ardb@kernel.org, akpm@linux-foundation.org, gshan@redhat.com, aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com, mochs@nvidia.com, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2 1/1] KVM: arm64: allow the VM to select DEVICE_* and NORMAL_NC for IO memory Message-ID: References: <20231205033015.10044-1-ankita@nvidia.com> <86fs0hatt3.wl-maz@kernel.org> <20231205130517.GD2692119@nvidia.com> <20231205164318.GG2692119@nvidia.com> <20231205194822.GL2692119@nvidia.com> <20231206150556.GQ2692119@nvidia.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=us-ascii Content-Disposition: inline In-Reply-To: <20231206150556.GQ2692119@nvidia.com> On Wed, Dec 06, 2023 at 11:05:56AM -0400, Jason Gunthorpe wrote: > On Wed, Dec 06, 2023 at 02:49:02PM +0000, Catalin Marinas wrote: > > On Tue, Dec 05, 2023 at 03:48:22PM -0400, Jason Gunthorpe wrote: > > > On Tue, Dec 05, 2023 at 07:24:37PM +0000, Catalin Marinas wrote: > > > > On Tue, Dec 05, 2023 at 12:43:18PM -0400, Jason Gunthorpe wrote: > > > > > What if we change vfio-pci to use pgprot_device() like it already > > > > > really should and say the pgprot_noncached() is enforced as > > > > > DEVICE_nGnRnE and pgprot_device() may be DEVICE_nGnRE or NORMAL_NC? > > > > > Would that be acceptable? > > > > > > > > pgprot_device() needs to stay as Device, otherwise you'd get speculative > > > > reads with potential side-effects. > > > > > > I do not mean to change pgprot_device() I mean to detect the > > > difference via pgprot_device() vs pgprot_noncached(). They put a > > > different value in the PTE that we can sense. It is very hacky. > > > > Ah, ok, it does look hacky though (as is the alternative of coming up > > with a new specific pgprot_*() that KVM can treat differently). > > > > BTW, on those Mellanox devices that require different attributes within > > a BAR, do they have a problem with speculative reads causing > > side-effects? > > Yes. We definitely have had that problem in the past on older > devices. VFIO must map the BAR using pgprot_device/noncached() into > the VMM, no other choice is functionally OK. Were those BARs tagged as prefetchable or non-prefetchable ? I assume the latter but please let me know if I am guessing wrong. Thanks, Lorenzo