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envelope-from=steven.sistare@oracle.com; helo=mx0b-00069f02.pphosted.com X-Spam_score_int: -27 X-Spam_score: -2.8 X-Spam_bar: -- X-Spam_report: (-2.8 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_MED=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 2/24/2025 2:20 PM, Peter Xu wrote: > On Mon, Feb 24, 2025 at 11:50:50AM -0500, Steven Sistare wrote: >>>> I can do that, but it would add cycles. Is this considered a high performance >>>> path that may be called frequently? >>> >>> AFAICT, any vIOMMU mapping isn't high perf path. In this specific path, >>> the refcount op should be buried in any dma map operations.. >> >> memory_region_ref contains a comment that implies we should avoid taking a >> ref if possible: >> * Memory regions without an owner are supposed to never go away; >> * we do not ref/unref them because it slows down DMA sensibly. > > That's for internal / permanent MRs that don't have an owner to speed > things up, and that should be orthogonal to this, as that'll automatically > take effect even if we use memory_region_[un]ref() here. Yes, I understand that is a different type of mr, but my point is that the author claims that memory_region_[un]ref() can noticeably slow DMA. > AFAIU we should always suggest using memory_region_[un]ref() in callers > when there's MR references. > > I'm also ok without the boosted refcount, but then please document the RCU > implications on referencing the MR, and the MR reference must not be used > after rcu_read_unlock(), in that case. I'll do that, to avoid any possibility of performance regression. - Steve