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X-Received-From: 2a00:1450:4864:20::442 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: , Cc: "qemu-devel@nongnu.org" , "Michael S. Tsirkin" Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" CCing qemu-devel On 7/1/19 11:57 AM, Igor Mammedov wrote: > On Mon, 1 Jul 2019 10:04:01 +0300 > Marcel Apfelbaum wrote: > >> 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 > Hi Marcel, > > What about my question about > sltsta = (sltsta & ~PCIE_SLOT_EVENTS) | (slt_sta & PCIE_SLOT_EVENTS); > being nop like > sltsta = LOWER_PART(sltsta) | UPPER_PART(sltsta) > > Can you point out what I'm missing here? Oops, I missed that, maybe it should look like:     if (...) {       sltsta = (val & ~PCIE_SLOT_EVENTS) | (sltsta & PCIE_SLOT_EVENTS);       ....   } to keep the PCIE_SLOT_EVENTS that were set before the write. Thanks, Marcel >> Thanks, >> Marcel >>