[PATCH v2] PCI: avoid bridge feature re-probing on hotplug

[Michael S. Tsirkin]

commit 1f82de10d6b1 ("PCI/x86: don't assume prefetchable ranges are 64bit")
added probing of bridge support for 64 bit memory each time bridge is
re-enumerated.

Unfortunately this probing is destructive if any device behind
the bridge is in use at this time.

This was observed in the field, see
https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg01711.html
and specifically
https://lists.gnu.org/archive/html/qemu-devel/2018-12/msg02082.html

There's no real need to re-probe the bridge features as the
registers in question never change - detect that using
the memory flag being set (it's always set on the 1st pass since
all PCI2PCI bridges support memory forwarding) and skip the probing.
Thus, only the first call will perform the disruptive probing and sets
the resource flags as required - which we can be reasonably sure happens
before any devices have been configured.
Avoiding repeated calls to pci_bridge_check_ranges might be even nicer.
Unfortunately I couldn't come up with a clean way to do it without a
major probing code refactoring.

Reported-by: xuyandong <xuyandong2@xxxxxxxxxx>
Tested-by: xuyandong <xuyandong2@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Cc: Yinghai Lu <yinghai@xxxxxxxxxx>
Cc: Jesse Barnes <jbarnes@xxxxxxxxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
---

Please review and consider for stable.

changes from v1:
	comment and commit log updates to address comments by Bjorn.

 drivers/pci/setup-bus.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/pci/setup-bus.c b/drivers/pci/setup-bus.c
index ed960436df5e..d5c25d465d97 100644
--- a/drivers/pci/setup-bus.c
+++ b/drivers/pci/setup-bus.c
@@ -741,6 +741,16 @@ static void pci_bridge_check_ranges(struct pci_bus *bus)
 	struct resource *b_res;
 
 	b_res = &bridge->resource[PCI_BRIDGE_RESOURCES];
+
+	/*
+	 * Don't re-check after this was called once already:
+	 * important since bridge might be in use.
+	 * Note: this is only reliable because as per spec all PCI to PCI
+	 * bridges support memory unconditionally so IORESOURCE_MEM is set.
+	 */
+	if (b_res[1].flags & IORESOURCE_MEM)
+		return;
+
 	b_res[1].flags |= IORESOURCE_MEM;
 
 	pci_read_config_word(bridge, PCI_IO_BASE, &io);
-- 
MST