PCI: Batch BAR sizing operations

Toggling memory enable is free on bare metal, but potentially expensive
in virtualized environments as the device MMIO spaces are added and
removed from the VM address space, including DMA mapping of those spaces
through the IOMMU where peer-to-peer is supported.  Currently memory
decode is disabled around sizing each individual BAR, even for SR-IOV
BARs while VF Enable is cleared.

This can be better optimized for virtual environments by sizing a set
of BARs at once, stashing the resulting mask into an array, while only
toggling memory enable once.  This also naturally improves the SR-IOV
path as the caller becomes responsible for any necessary decode disables
while sizing BARs, therefore SR-IOV BARs are sized relying only on the
VF Enable rather than toggling the PF memory enable in the command
register.

Link: https://lore.kernel.org/r/20250120182202.1878581-1-alex.williamson@redhat.com
Reported-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Link: https://lore.kernel.org/r/CAHTA-uYp07FgM6T1OZQKqAdSA5JrZo0ReNEyZgQZub4mDRrV5w@mail.gmail.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Tested-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Reviewed-by: Mitchell Augustin <mitchell.augustin@canonical.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

1 files changed, 68 insertions, 25 deletions

diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
index 2e81ab0f5a25..bf6aec555044 100644
--- a/drivers/pci/probe.c
+++ b/drivers/pci/probe.c
@@ -165,40 +165,66 @@ static inline unsigned long decode_bar(struct pci_dev *dev, u32 bar)
 #define PCI_COMMAND_DECODE_ENABLE	(PCI_COMMAND_MEMORY | PCI_COMMAND_IO)
 
 /**
+ * __pci_size_bars - Read the raw BAR mask for a range of PCI BARs
+ * @dev: the PCI device
+ * @count: number of BARs to size
+ * @pos: starting config space position
+ * @sizes: array to store mask values
+ * @rom: indicate whether to use ROM mask, which avoids enabling ROM BARs
+ *
+ * Provided @sizes array must be sufficiently sized to store results for
+ * @count u32 BARs.  Caller is responsible for disabling decode to specified
+ * BAR range around calling this function.  This function is intended to avoid
+ * disabling decode around sizing each BAR individually, which can result in
+ * non-trivial overhead in virtualized environments with very large PCI BARs.
+ */
+static void __pci_size_bars(struct pci_dev *dev, int count,
+			    unsigned int pos, u32 *sizes, bool rom)
+{
+	u32 orig, mask = rom ? PCI_ROM_ADDRESS_MASK : ~0;
+	int i;
+
+	for (i = 0; i < count; i++, pos += 4, sizes++) {
+		pci_read_config_dword(dev, pos, &orig);
+		pci_write_config_dword(dev, pos, mask);
+		pci_read_config_dword(dev, pos, sizes);
+		pci_write_config_dword(dev, pos, orig);
+	}
+}
+
+void __pci_size_stdbars(struct pci_dev *dev, int count,
+			unsigned int pos, u32 *sizes)
+{
+	__pci_size_bars(dev, count, pos, sizes, false);
+}
+
+static void __pci_size_rom(struct pci_dev *dev, unsigned int pos, u32 *sizes)
+{
+	__pci_size_bars(dev, 1, pos, sizes, true);
+}
+
+/**
  * __pci_read_base - Read a PCI BAR
  * @dev: the PCI device
  * @type: type of the BAR
  * @res: resource buffer to be filled in
  * @pos: BAR position in the config space
+ * @sizes: array of one or more pre-read BAR masks
  *
  * Returns 1 if the BAR is 64-bit, or 0 if 32-bit.
  */
 int __pci_read_base(struct pci_dev *dev, enum pci_bar_type type,
-		    struct resource *res, unsigned int pos)
+		    struct resource *res, unsigned int pos, u32 *sizes)
 {
-	u32 l = 0, sz = 0, mask;
+	u32 l = 0, sz;
 	u64 l64, sz64, mask64;
-	u16 orig_cmd;
 	struct pci_bus_region region, inverted_region;
 	const char *res_name = pci_resource_name(dev, res - dev->resource);
 
-	mask = type ? PCI_ROM_ADDRESS_MASK : ~0;
-
-	/* No printks while decoding is disabled! */
-	if (!dev->mmio_always_on) {
-		pci_read_config_word(dev, PCI_COMMAND, &orig_cmd);
-		if (orig_cmd & PCI_COMMAND_DECODE_ENABLE) {
-			pci_write_config_word(dev, PCI_COMMAND,
-				orig_cmd & ~PCI_COMMAND_DECODE_ENABLE);
-		}
-	}
-
 	res->name = pci_name(dev);
 
 	pci_read_config_dword(dev, pos, &l);
-	pci_write_config_dword(dev, pos, l | mask);
-	pci_read_config_dword(dev, pos, &sz);
-	pci_write_config_dword(dev, pos, l);
+	sz = sizes[0];
 
 	/*
 	 * All bits set in sz means the device isn't working properly.
@@ -238,18 +264,13 @@ int __pci_read_base(struct pci_dev *dev, enum pci_bar_type type,
 
 	if (res->flags & IORESOURCE_MEM_64) {
 		pci_read_config_dword(dev, pos + 4, &l);
-		pci_write_config_dword(dev, pos + 4, ~0);
-		pci_read_config_dword(dev, pos + 4, &sz);
-		pci_write_config_dword(dev, pos + 4, l);
+		sz = sizes[1];
 
 		l64 |= ((u64)l << 32);
 		sz64 |= ((u64)sz << 32);
 		mask64 |= ((u64)~0 << 32);
 	}
 
-	if (!dev->mmio_always_on && (orig_cmd & PCI_COMMAND_DECODE_ENABLE))
-		pci_write_config_word(dev, PCI_COMMAND, orig_cmd);
-
 	if (!sz64)
 		goto fail;
 
@@ -320,7 +341,11 @@ out:
 
 static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
 {
+	u32 rombar, stdbars[PCI_STD_NUM_BARS];
 	unsigned int pos, reg;
+	u16 orig_cmd;
+
+	BUILD_BUG_ON(howmany > PCI_STD_NUM_BARS);
 
 	if (dev->non_compliant_bars)
 		return;
@@ -329,10 +354,28 @@ static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
 	if (dev->is_virtfn)
 		return;
 
+	/* No printks while decoding is disabled! */
+	if (!dev->mmio_always_on) {
+		pci_read_config_word(dev, PCI_COMMAND, &orig_cmd);
+		if (orig_cmd & PCI_COMMAND_DECODE_ENABLE) {
+			pci_write_config_word(dev, PCI_COMMAND,
+				orig_cmd & ~PCI_COMMAND_DECODE_ENABLE);
+		}
+	}
+
+	__pci_size_stdbars(dev, howmany, PCI_BASE_ADDRESS_0, stdbars);
+	if (rom)
+		__pci_size_rom(dev, rom, &rombar);
+
+	if (!dev->mmio_always_on &&
+	    (orig_cmd & PCI_COMMAND_DECODE_ENABLE))
+		pci_write_config_word(dev, PCI_COMMAND, orig_cmd);
+
 	for (pos = 0; pos < howmany; pos++) {
 		struct resource *res = &dev->resource[pos];
 		reg = PCI_BASE_ADDRESS_0 + (pos << 2);
-		pos += __pci_read_base(dev, pci_bar_unknown, res, reg);
+		pos += __pci_read_base(dev, pci_bar_unknown,
+				       res, reg, &stdbars[pos]);
 	}
 
 	if (rom) {
@@ -340,7 +383,7 @@ static void pci_read_bases(struct pci_dev *dev, unsigned int howmany, int rom)
 		dev->rom_base_reg = rom;
 		res->flags = IORESOURCE_MEM | IORESOURCE_PREFETCH |
 				IORESOURCE_READONLY | IORESOURCE_SIZEALIGN;
-		__pci_read_base(dev, pci_bar_mem32, res, rom);
+		__pci_read_base(dev, pci_bar_mem32, res, rom, &rombar);
 	}
 }