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[54.240.197.228]) by smtp.gmail.com with ESMTPSA id l10-20020adfe58a000000b003434f526cb5sm8864955wrm.95.2024.04.08.06.10.36 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 08 Apr 2024 06:10:37 -0700 (PDT) From: Paul Durrant X-Google-Original-From: Paul Durrant Message-ID: <60c49e27-1275-411e-9cda-8405ca0ce010@xen.org> Date: Mon, 8 Apr 2024 14:10:36 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xen-hvm: Avoid livelock while handling buffered ioreqs To: Ross Lagerwall Cc: Stefano Stabellini , Anthony Perard , xen-devel@lists.xenproject.org, qemu-devel@nongnu.org References: <20240404140833.1557953-1-ross.lagerwall@citrix.com> <6fad8151-4bde-4a02-96fb-27b99d93757f@gmail.com> Content-Language: en-US Organization: Xen Project In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=2a00:1450:4864:20::434; envelope-from=xadimgnik@gmail.com; helo=mail-wr1-x434.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, RCVD_IN_DNSWL_NONE=-0.0001, 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: , Reply-To: paul@xen.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 08/04/2024 14:00, Ross Lagerwall wrote: > On Sat, Apr 6, 2024 at 11:58 AM Durrant, Paul wrote: >> >> On 04/04/2024 15:08, Ross Lagerwall wrote: >>> A malicious or buggy guest may generated buffered ioreqs faster than >>> QEMU can process them in handle_buffered_iopage(). The result is a >>> livelock - QEMU continuously processes ioreqs on the main thread without >>> iterating through the main loop which prevents handling other events, >>> processing timers, etc. Without QEMU handling other events, it often >>> results in the guest becoming unsable and makes it difficult to stop the >>> source of buffered ioreqs. >>> >>> To avoid this, if we process a full page of buffered ioreqs, stop and >>> reschedule an immediate timer to continue processing them. This lets >>> QEMU go back to the main loop and catch up. >>> >> >> Do PV backends potentially cause the same scheduling issue (if not using >> io threads)? >> > > From what I can tell: > > xen-block: It reads req_prod / req_cons once before entering the loop > so it should be fine, I think. > > xen_console: Same as xen-block > > xen_nic: It reads req_prod / req_cons once before entering the loop. > However, once the loop ends it checks for more requests and if there > are more requests it restarts from the beginning. It seems like this > could be susceptible to the same issue. > > (These PV backends generally aren't used by XenServer's system QEMU > so I didn't spend too much time looking into it.) > > Thanks, Ok. Thanks for checking. Paul