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Mon, 29 Jun 2020 14:31:08 +0300 Date: Mon, 29 Jun 2020 14:31:07 +0300 From: Roman Bolshakov To: Paolo Bonzini Subject: Re: [PATCH 4/8] i386: hvf: Implement CPU kick Message-ID: <20200629113107.GL25104@SPB-NB-133.local> References: <20200624225850.16982-1-r.bolshakov@yadro.com> <20200624225850.16982-5-r.bolshakov@yadro.com> <9d63db4e-25a7-9c77-6f05-e5f808b8b33c@redhat.com> <20200625155712.GI25104@SPB-NB-133.local> <6508d40b-0142-1b42-2f48-fcd2da66ea4b@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Disposition: inline In-Reply-To: <6508d40b-0142-1b42-2f48-fcd2da66ea4b@redhat.com> X-Originating-IP: [172.17.204.212] X-ClientProxiedBy: T-EXCH-01.corp.yadro.com (172.17.10.101) To T-EXCH-02.corp.yadro.com (172.17.10.102) Received-SPF: pass client-ip=89.207.88.252; envelope-from=r.bolshakov@yadro.com; helo=mta-01.yadro.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/06/29 07:31:11 X-ACL-Warn: Detected OS = Linux 3.11 and newer 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, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Claudio Fontana , Richard Henderson , Eduardo Habkost , Cameron Esfahani , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Jun 25, 2020 at 08:34:14PM +0200, Paolo Bonzini wrote: > On 25/06/20 17:57, Roman Bolshakov wrote: > > So, the kick is not delivered to self and in case if destination cpu is > > not running. I think it can't interrupt subsequent hv_vcpu_run. > > Yes. > > >> If not, you can reduce a bit the race window by setting a variable in > >> cpu, like > >> > >> atomic_set(&cpu->deadline, 0); > >> hv_vcpu_interrupt(...) > >> > >> and in the vCPU thread > >> > >> hv_vcpu_run_until(..., atomic_read(&cpu->deadline)); > >> atomic_set(&cpu->deadline, HV_DEADLINE_FOREVER); > >> > > > > Sure, could you please explain who'll be racing? There's a race if a > > kick was sent after VMEXIT, right? So essentially we need a way to > > "requeue" a kick that was received outside of hv_vcpu_run to avoid loss > > of it? > > Yes. Note that this is not a new bug, it's pre-existing and it's common > to all hypervisors except KVM/WHPX. I mean not the QEMU code, it's the > kernel APIs that are broken. :) > > One way to do so is to keep the signal, and have the signal handler > enable the preemption timer (with a deadline of 0) in the pin-based > interrupt controls. Hopefully macOS allows that, especially on 10.15+ > where hv_vcpu_run_until probably uses the preemption timer. > > > hv_vcpu_run_until is only available on macOS 10.15+ and we can't use yet > > because of three release support rule. > > (https://developer.apple.com/documentation/hypervisor/3181548-hv_vcpu_run_until?language=objc) > > > > BTW, I'm totally okay to send v2 if kicks are lost and/or the patch > > needs improvements. (and I can address EFER to VMCS Entry Controls > > synchronization as well) > > > > Paolo, do you know any particular test in kvm-unit-tests that can > > exhibit the issue? > > No, it's a race and it's extremely rare, but I point it out because it's > a kernel issue that Apple might want to fix anyway. It might also be > (depending on how the kernel side is written) that the next scheduler > tick will end up unblocking the vCPU and papering over it. > Hi Paolo, I implemented what you proposed using VMX-preemption timer in Pin-based controls and regular hv_vcpu_run(). It works fine without noticable regressions, I'll send that in v2. hv_vcpu_run_until() was also evaluated on macOS 10.15.5 but it degrades VM performance significantly compared to explicit setting of VMX-preepmtion timer value and hv_vcpu_run(). The performance issue was observed on Broadwell-based MacBook Air and Ivy Bridge-based MacBook Pro. macOS 11.0 Beta deprecated hv_vcpu_run() and introduced a special declaration for hv_vcpu_run_until(), that's not available 10.15 - HV_DEADLINE_FOREVER (UINT64_MAX, which is bigger than maximum value of VMX-preeemption counter). Perhaps the performance issue is addressed there. Regards, Roman