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[34.127.9.76]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-36490dd6be5sm347273a91.6.2026.04.27.14.16.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 27 Apr 2026 14:16:16 -0700 (PDT) Date: Mon, 27 Apr 2026 21:16:12 +0000 From: David Matlack To: Jacob Pan Cc: iommu@lists.linux.dev, kexec@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-pci@vger.kernel.org, Adithya Jayachandran , Alexander Graf , Alex Williamson , Bjorn Helgaas , Chris Li , David Rientjes , Jason Gunthorpe , Joerg Roedel , Jonathan Corbet , Josh Hilke , Leon Romanovsky , Lukas Wunner , Mike Rapoport , Parav Pandit , Pasha Tatashin , Pranjal Shrivastava , Pratyush Yadav , Robin Murphy , Saeed Mahameed , Samiullah Khawaja , Shuah Khan , Will Deacon , William Tu , Yi Liu Subject: Re: [PATCH v4 05/11] PCI: liveupdate: Inherit bus numbers during Live Update Message-ID: References: <20260423212316.3431746-1-dmatlack@google.com> <20260423212316.3431746-6-dmatlack@google.com> <20260427114745.00000656@linux.microsoft.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On 2026-04-27 08:40 PM, David Matlack wrote: > On 2026-04-27 11:47 AM, Jacob Pan wrote: > > On Thu, 23 Apr 2026 21:23:09 +0000 > > David Matlack wrote: > > > > To keep things simple, inherit the secondary and subordinate bus > > > numbers on all bridges if any PCI devices were preserved (i.e. even > > > bridges without any downstream endpoints that were preserved). This > > > avoids accidentally assigning a bridge a new window that overlaps > > > with a preserved device that is downstream of a different bridge. > > > > > > If a bridge is enumerated with a broken topology or has no bus numbers > > > set during a Live Update, refuse to assign it new bus numbers and > > > refuse to enumerate devices below it. This is a safety measure to > > > prevent topology conflicts. > > > > > > Require that CONFIG_CARDBUS is not enabled to enable > > > CONFIG_PCI_LIVEUPDATE since inheriting bus numbers on PCI-to-CardBus > > > bridges requires additional work but is not a priority at the moment. > > > > > > Signed-off-by: David Matlack > > > > + /* > > > + * During a Live Update, preserved devices are allowed to > > > continue > > > + * performing memory transactions. The kernel must not > > > change the fabric > > > + * topology, including bus numbers, since that would require > > > disabling > > > + * and flushing any memory transactions first. > > > + * > > > + * To keep things simple, inherit the secondary and > > > subordinate bus > > > + * numbers on _all_ bridges if _any_ PCI devices were > > > preserved (i.e. > > > + * even bridges without any downstream endpoints that were > > > preserved). > > > + * This avoids accidentally assigning a bridge a new window > > > that > > > + * overlaps with a preserved device that is downstream of a > > > different > > > + * bridge. > > > + */ > > > + dev->liveupdate_inherit_buses = true; > > > + > > This flag never gets cleared after the incoming kernel boot up, what if > > the user does a manual rescan via sysfs? i.e. > > # echo 1 > /sys/bus/pci/rescan > > pcibios_assign_all_busses() will never gets called for this device, and > > may hit this > > if (dev->liveupdate_inherit_buses) { > > pci_err(dev, "Cannot reconfigure bridge during > > Live Update!\n"); > > > > So, maybe clear it in pci_liveupdate_finish()? > > I think we can allo wa rescan to assign new bus numbers once all devices > go through pci_liveupdate_finish() by clearing > dev->liveupdate_inherit_buses on all devices in pci_flb_finish(). We > would need to hold pci_rescan_remove_lock to avoid this racing with such > a rescan. > > Now that you bring up /sys/bus/pci/rescan... I think we also need to > set dev->liveupdate_inherit_buses in the outgoing kernel, to avoid bus > numbers changing on outgoing preserved devices. pci_flb_preserve() > should take pci_rescan_remove_lock and set dev->liveupdate_inherit_buses > on all devices, and pci_flb_unpreserve() should do the opposite. > > If we did all then then /sys/bus/pci/rescan can work like normal as long > as no devices are preserved (incoming or outgoing). If any devices are > preserved then dev->liveupdate_inherit_buses gets set to prevent bus > numbers from changing during a possible rescan. Something like this? This is a diff applied on top of this commit. diff --git a/drivers/pci/liveupdate.c b/drivers/pci/liveupdate.c index fead478e8a04..b1b0a5b1a5df 100644 --- a/drivers/pci/liveupdate.c +++ b/drivers/pci/liveupdate.c @@ -120,6 +120,20 @@ #include "pci.h" +/* + * During a Live Update, preserved devices are allowed to continue performing + * memory transactions. The kernel must not change the fabric topology, + * including bus numbers, since that would require disabling and flushing any + * memory transactions first. + * + * To keep things simple, inherit the secondary and subordinate bus numbers on + * _all_ bridges if _any_ PCI devices are preserved (i.e. even bridges without + * any downstream endpoints that were preserved). This avoids accidentally + * assigning a bridge a new window that overlaps with a preserved device that is + * downstream of a different bridge. + */ +static atomic_t inherit_buses; + struct pci_flb_outgoing { /* The pci_ser struct to be passed to the next kernel */ struct pci_ser *ser; @@ -141,6 +155,29 @@ static unsigned long pci_ser_xa_key(unsigned long domain, unsigned long bdf) return domain << 16 | bdf; } +bool pci_liveupdate_inherit_buses(void) +{ + return atomic_read(&inherit_buses); +} + +static void pci_set_liveupdate_inherit_buses(bool enable) +{ + /* Ensure updates to inherit_buses do not race with rescans */ + pci_lock_rescan_remove(); + + /* + * Increment/decrement instead of setting directly to true/false so that + * pci_liveupdate_inherit_buses() returns true if any device is outgoing + * preserved or incoming preserved. + */ + if (enable) + atomic_inc(&inherit_buses); + else + atomic_dec(&inherit_buses); + + pci_unlock_rescan_remove(); +} + static int pci_flb_preserve(struct liveupdate_flb_op_args *args) { struct pci_flb_outgoing *outgoing; @@ -180,6 +217,8 @@ static int pci_flb_preserve(struct liveupdate_flb_op_args *args) args->obj = outgoing; args->data = virt_to_phys(outgoing->ser); + + pci_set_liveupdate_inherit_buses(true); return 0; } @@ -187,6 +226,8 @@ static void pci_flb_unpreserve(struct liveupdate_flb_op_args *args) { struct pci_flb_outgoing *outgoing = args->obj; + pci_set_liveupdate_inherit_buses(false); + pr_debug("Unpreserving struct pci_ser\n"); WARN_ON_ONCE(outgoing->ser->nr_devices); kho_unpreserve_free(outgoing->ser); @@ -223,6 +264,8 @@ static int pci_flb_retrieve(struct liveupdate_flb_op_args *args) } args->obj = incoming; + + pci_set_liveupdate_inherit_buses(true); return 0; } @@ -230,6 +273,8 @@ static void pci_flb_finish(struct liveupdate_flb_op_args *args) { struct pci_flb_incoming *incoming = args->obj; + pci_set_liveupdate_inherit_buses(false); + xa_destroy(&incoming->xa); kho_restore_free(incoming->ser); kfree(incoming); @@ -385,21 +430,6 @@ void pci_liveupdate_setup_device(struct pci_dev *dev) if (!xa) return; - /* - * During a Live Update, preserved devices are allowed to continue - * performing memory transactions. The kernel must not change the fabric - * topology, including bus numbers, since that would require disabling - * and flushing any memory transactions first. - * - * To keep things simple, inherit the secondary and subordinate bus - * numbers on _all_ bridges if _any_ PCI devices were preserved (i.e. - * even bridges without any downstream endpoints that were preserved). - * This avoids accidentally assigning a bridge a new window that - * overlaps with a preserved device that is downstream of a different - * bridge. - */ - dev->liveupdate_inherit_buses = true; - key = pci_ser_xa_key(pci_domain_nr(dev->bus), pci_dev_id(dev)); dev_ser = xa_load(xa, key); diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h index 09bab39738d7..abd8379b99cf 100644 --- a/drivers/pci/pci.h +++ b/drivers/pci/pci.h @@ -1442,6 +1442,7 @@ static inline int pci_msix_write_tph_tag(struct pci_dev *pdev, unsigned int inde #ifdef CONFIG_PCI_LIVEUPDATE void pci_liveupdate_setup_device(struct pci_dev *dev); void pci_liveupdate_cleanup_device(struct pci_dev *dev); +bool pci_liveupdate_inherit_buses(void); #else static inline void pci_liveupdate_setup_device(struct pci_dev *dev) { @@ -1450,6 +1451,11 @@ static inline void pci_liveupdate_setup_device(struct pci_dev *dev) static inline void pci_liveupdate_cleanup_device(struct pci_dev *dev) { } + +static inline bool pci_liveupdate_inherit_buses(void) +{ + return false; +} #endif #endif /* DRIVERS_PCI_H */ diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c index fa26f4170add..f94fa1fc76cc 100644 --- a/drivers/pci/probe.c +++ b/drivers/pci/probe.c @@ -1374,9 +1374,9 @@ bool pci_ea_fixed_busnrs(struct pci_dev *dev, u8 *sec, u8 *sub) return true; } -static bool pci_should_assign_new_buses(struct pci_dev *dev) +static bool pci_should_assign_new_buses(void) { - if (dev->liveupdate_inherit_buses) + if (pci_liveupdate_inherit_buses()) return false; return pcibios_assign_all_busses(); @@ -1409,7 +1409,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, int max, unsigned int available_buses, int pass) { - const bool assign_new_buses = pci_should_assign_new_buses(dev); + const bool assign_new_buses = pci_should_assign_new_buses(); struct pci_bus *child; u32 buses; u16 bctl; @@ -1518,7 +1518,7 @@ static int pci_scan_bridge_extend(struct pci_bus *bus, struct pci_dev *dev, goto out; } - if (dev->liveupdate_inherit_buses) { + if (pci_liveupdate_inherit_buses()) { pci_err(dev, "Cannot reconfigure bridge during Live Update!\n"); pci_err(dev, "Downstream devices will not be enumerated!\n"); goto out; diff --git a/include/linux/pci.h b/include/linux/pci.h index 9a602b322e3c..dd6b26ca9462 100644 --- a/include/linux/pci.h +++ b/include/linux/pci.h @@ -511,7 +511,6 @@ struct pci_dev { unsigned int rom_bar_overlap:1; /* ROM BAR disable broken */ unsigned int rom_attr_enabled:1; /* Display of ROM attribute enabled? */ unsigned int non_mappable_bars:1; /* BARs can't be mapped to user-space */ - unsigned int liveupdate_inherit_buses:1; /* Inherit bus numbers due to Live Update */ pci_dev_flags_t dev_flags; atomic_t enable_cnt; /* pci_enable_device has been called */