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Mon, 23 Mar 2020 08:25:27 +0000 MIME-Version: 1.0 Date: Mon, 23 Mar 2020 08:25:27 +0000 From: Marc Zyngier To: Zenghui Yu Subject: Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation selection in the distributor In-Reply-To: References: <20200304203330.4967-1-maz@kernel.org> <20200304203330.4967-21-maz@kernel.org> <72832f51-bbde-8502-3e03-189ac20a0143@huawei.com> <4a06fae9c93e10351276d173747d17f4@kernel.org> <1c9fdfc8-bdb2-88b6-4bdc-2b9254dfa55c@huawei.com> <256b58a9679412c96600217f316f424f@kernel.org> Message-ID: <1c10593ac5b75f37c6853fbc74daa481@kernel.org> X-Sender: maz@kernel.org User-Agent: Roundcube Webmail/1.3.10 X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: yuzenghui@huawei.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, lorenzo.pieralisi@arm.com, jason@lakedaemon.net, rrichter@marvell.com, tglx@linutronix.de, eric.auger@redhat.com, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); 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Mon, 23 Mar 2020 08:25:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1584951929; bh=tqQ8PlsQArzeFElGfgu0kJhz7Czy0eqAc18ZQGW6JD8=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=tZ4grYy2alHlUvPC7sxjJFVOb1mrLYtqyhRfBHfGnbDbVuviyeian+drdS/c1uww+ 6ZKBvhncRTZ8u0v/xdXoKvOz3n5fT6bjzdUDR4GPsILVepol0jbF2YhVx4JwV9nDtv 5UaxhARaZQ4WTIaDKxaZmT0MmNSNNhz2LBe5rROs= Received: from disco-boy.misterjones.org ([51.254.78.96] helo=www.loen.fr) by disco-boy.misterjones.org with esmtpsa (TLS1.2:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1jGIOl-00Etsi-6T; Mon, 23 Mar 2020 08:25:27 +0000 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Date: Mon, 23 Mar 2020 08:25:27 +0000 From: Marc Zyngier To: Zenghui Yu Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Lorenzo Pieralisi , Jason Cooper , Robert Richter , Thomas Gleixner , Eric Auger , James Morse , Julien Thierry , Suzuki K Poulose Subject: Re: [PATCH v5 20/23] KVM: arm64: GICv4.1: Plumb SGI implementation selection in the distributor In-Reply-To: References: <20200304203330.4967-1-maz@kernel.org> <20200304203330.4967-21-maz@kernel.org> <72832f51-bbde-8502-3e03-189ac20a0143@huawei.com> <4a06fae9c93e10351276d173747d17f4@kernel.org> <1c9fdfc8-bdb2-88b6-4bdc-2b9254dfa55c@huawei.com> <256b58a9679412c96600217f316f424f@kernel.org> Message-ID: <1c10593ac5b75f37c6853fbc74daa481@kernel.org> X-Sender: maz@kernel.org User-Agent: Roundcube Webmail/1.3.10 X-SA-Exim-Connect-IP: 51.254.78.96 X-SA-Exim-Rcpt-To: yuzenghui@huawei.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, lorenzo.pieralisi@arm.com, jason@lakedaemon.net, rrichter@marvell.com, tglx@linutronix.de, eric.auger@redhat.com, james.morse@arm.com, julien.thierry.kdev@gmail.com, suzuki.poulose@arm.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org Hi Zenghui, [...] >> And actually, maybe we can handle that pretty cheaply. If userspace >> tries to restore GICD_TYPER2 to a value that isn't what KVM can >> offer, we just return an error. Exactly like we do for GICD_IIDR. >> Hence the following patch: >> >> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c >> b/virt/kvm/arm/vgic/vgic-mmio-v3.c >> index 28b639fd1abc..e72dcc454247 100644 >> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c >> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c >> @@ -156,6 +156,7 @@ static int vgic_mmio_uaccess_write_v3_misc(struct >> kvm_vcpu *vcpu, >>      struct vgic_dist *dist = &vcpu->kvm->arch.vgic; >> >>      switch (addr & 0x0c) { >> +    case GICD_TYPER2: >>      case GICD_IIDR: >>          if (val != vgic_mmio_read_v3_misc(vcpu, addr, len)) >>              return -EINVAL; >> >> Being a RO register, writing something that isn't compatible with the >> possible behaviour of the hypervisor will just return an error. > > This is really a nice point to address my concern! I'm happy to see > this in v6 now. > >> >> What do you think? > > I agreed with you, with a bit nervous though. Some old guests (which > have no knowledge about GICv4.1 vSGIs and don't care about nASSGIcap > at all) will also fail to migrate from A to B, just because now we > present two different (unused) GICD_TYPER2 registers to them. > > Is it a little unfair to them :-) ? I never pretended to be fair! ;-) I'm happy to prevent migrating from a v4.1 system (A) to a v4.0 system (B). As soon as the guest has run, it isn't safe to do so (it may have read TYPER2, and now knows about vSGIs). We *could* track this and find ways to migrate this state as well, but it feels fragile. Migrating from B to A is more appealing. It should be possible to do so without much difficulty (just check that the nASSGIcap bit is either 0 or equal to KVM's view of the capability). But overall I seriously doubt we can easily migrate guests across very different HW. We've been talking about this for years, and we still don't have a good solution for it given the diversity of the ecosystem... :-/ Thanks, M. -- Jazz is not dead. It just smells funny...