From mboxrd@z Thu Jan 1 00:00:00 1970 From: Joerg Roedel Subject: Re: [PATCH v6 26/34] iommu/amd: Allow the AMD IOMMU to work with memory encryption Date: Wed, 21 Jun 2017 17:37:22 +0200 Message-ID: <20170621153721.GP30388@8bytes.org> References: <20170607191309.28645.15241.stgit@tlendack-t1.amdoffice.net> <20170607191745.28645.81756.stgit@tlendack-t1.amdoffice.net> <20170614174208.p2yr5exs4b6pjxhf@pd.tnic> <0611d01a-19f8-d6ae-2682-932789855518@amd.com> <20170615094111.wga334kg2bhxqib3@pd.tnic> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20170615094111.wga334kg2bhxqib3@pd.tnic> Sender: owner-linux-mm@kvack.org To: Borislav Petkov Cc: Tom Lendacky , linux-arch@vger.kernel.org, linux-efi@vger.kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, x86@kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com, linux-mm@kvack.org, iommu@lists.linux-foundation.org, Rik van Riel , Radim =?utf-8?B?S3LEjW3DocWZ?= , Toshimitsu Kani , Arnd Bergmann , Jonathan Corbet , Matt Fleming , "Michael S. Tsirkin" , Konrad Rzeszutek Wilk , Paolo Bonzini , Larry Woodman , Brijesh Singh , Ingo Molnar , Andy Lutomirski , "H. Peter Anvin" , Andrey Ryabinin , Alexand List-Id: iommu@lists.linux-foundation.org On Thu, Jun 15, 2017 at 11:41:12AM +0200, Borislav Petkov wrote: > On Wed, Jun 14, 2017 at 03:40:28PM -0500, Tom Lendacky wrote: > > > WARNING: Use of volatile is usually wrong: see Documentation/process/volatile-considered-harmful.rst > > > #134: FILE: drivers/iommu/amd_iommu.c:866: > > > +static void build_completion_wait(struct iommu_cmd *cmd, volatile u64 *sem) > > > > > > > The semaphore area is written to by the device so the use of volatile is > > appropriate in this case. > > Do you mean this is like the last exception case in that document above: > > " > - Pointers to data structures in coherent memory which might be modified > by I/O devices can, sometimes, legitimately be volatile. A ring buffer > used by a network adapter, where that adapter changes pointers to > indicate which descriptors have been processed, is an example of this > type of situation." > > ? So currently (without this patch) the build_completion_wait function does not take a volatile parameter, only wait_on_sem() does. Wait_on_sem() needs it because its purpose is to poll a memory location which is changed by the iommu-hardware when its done with command processing. But the 'volatile' in build_completion_wait() looks unnecessary, because the function does not poll the memory location. It only uses the pointer, converts it to a physical address and writes it to the command to be queued. Regards, Joerg -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org