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Tsirkin" , qemu-devel@nongnu.org References: <20190621064615.20099-1-mst@redhat.com> <20190621064615.20099-4-mst@redhat.com> From: Marcel Apfelbaum Message-ID: <4e4314f7-91cc-1413-9853-bfd2bbb26311@gmail.com> Date: Mon, 1 Jul 2019 10:04:01 +0300 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190621064615.20099-4-mst@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US X-detected-operating-system: by eggs.gnu.org: Genre and OS details not recognized. X-Received-From: 2a00:1450:4864:20::443 Subject: Re: [Qemu-devel] [PATCH 3/3] pcie: work around for racy guest init 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: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 6/21/19 9:46 AM, Michael S. Tsirkin wrote: > During boot, linux guests tend to clear all bits in pcie slot status > register which is used for hotplug. > If they clear bits that weren't set this is racy and will lose events: > not a big problem for manual hotplug on bare-metal, but a problem for us. > > For example, the following is broken ATM: > > /x86_64-softmmu/qemu-system-x86_64 -enable-kvm -S -machine q35 \ > -device pcie-root-port,id=pcie_root_port_0,slot=2,chassis=2,addr=0x2,bus=pcie.0 \ > -device virtio-balloon-pci,id=balloon,bus=pcie_root_port_0 \ > -monitor stdio disk.qcow2 > (qemu)device_del balloon > (qemu)cont > > Balloon isn't deleted as it should. > > As a work-around, detect this attempt to clear slot status and revert > status to what it was before the write. > > Note: in theory this can be detected as a duplicate button press > which cancels the previous press. Does not seem to happen in > practice as guests seem to only have this bug during init. > > Note2: the right thing to do is probably to fix Linux to > read status before clearing it, and act on the bits that are set. > > Signed-off-by: Michael S. Tsirkin > --- > hw/pci/pcie.c | 19 +++++++++++++++++++ > 1 file changed, 19 insertions(+) > > diff --git a/hw/pci/pcie.c b/hw/pci/pcie.c > index f8490a00de..c605d32dd4 100644 > --- a/hw/pci/pcie.c > +++ b/hw/pci/pcie.c > @@ -610,6 +610,25 @@ void pcie_cap_slot_write_config(PCIDevice *dev, uint16_t slt_ctl, uint16_t slt_s > uint16_t sltsta = pci_get_word(exp_cap + PCI_EXP_SLTSTA); > > if (ranges_overlap(addr, len, pos + PCI_EXP_SLTSTA, 2)) { > + /* > + * Guests tend to clears all bits during init. > + * If they clear bits that weren't set this is racy and will lose events: > + * not a big problem for manual button presses, but a problem for us. > + * As a work-around, detect this and revert status to what it was > + * before the write. > + * > + * Note: in theory this can be detected as a duplicate button press > + * which cancels the previous press. Does not seem to happen in > + * practice as guests seem to only have this bug during init. > + */ > +#define PCIE_SLOT_EVENTS (PCI_EXP_SLTSTA_ABP | PCI_EXP_SLTSTA_PFD | \ > + PCI_EXP_SLTSTA_MRLSC | PCI_EXP_SLTSTA_PDC | \ > + PCI_EXP_SLTSTA_CC) > + > + if (val & ~slt_sta & PCIE_SLOT_EVENTS) { > + sltsta = (sltsta & ~PCIE_SLOT_EVENTS) | (slt_sta & PCIE_SLOT_EVENTS); > + pci_set_word(exp_cap + PCI_EXP_SLTSTA, sltsta); > + } > hotplug_event_clear(dev); > } > Reviewed-by: Marcel Apfelbaum Thanks, Marcel