Re: [PATCH v2] PCI: Batch BAR sizing operations

Ilpo Järvinen
2025-01-23T12:15:53Z

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On Mon, 20 Jan 2025, Alex Williamson wrote:

> 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.
>
> Reported-by: Mitchell Augustin <mitchell.augustin@canonical.com>
> Link: https://lore.kernel.org/all/CAHTA-uYp07FgM6T1OZQKqAdSA5JrZo0ReNEyZgQZub4mDRrV5w@mail.gmail.com
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> ---
>
> v2:
> - Move PCI_POSSIBLE_ERROR() test back to original location such that it
> only tests the lower half of 64-bit BARs as noted by Ilpo Järvinen.
> - Reduce delta from original code by retaining the local @sz variable
> filled from the @sizes array and keep location of parsing upper half
> of 64-bit BARs.

Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>

--
i.

> drivers/pci/iov.c | 8 +++-
> drivers/pci/pci.h | 4 +-
> drivers/pci/probe.c | 93 +++++++++++++++++++++++++++++++++------------
> 3 files changed, 78 insertions(+), 27 deletions(-)
>
> diff --git a/drivers/pci/iov.c b/drivers/pci/iov.c
> index 4be402fe9ab9..9e4770cdd4d5 100644
> --- a/drivers/pci/iov.c
> +++ b/drivers/pci/iov.c
> @@ -747,6 +747,7 @@ static int sriov_init(struct pci_dev *dev, int pos)
> struct resource *res;
> const char *res_name;
> struct pci_dev *pdev;
> + u32 sriovbars[PCI_SRIOV_NUM_BARS];
>
> pci_read_config_word(dev, pos + PCI_SRIOV_CTRL, &ctrl);
> if (ctrl & PCI_SRIOV_CTRL_VFE) {
> @@ -783,6 +784,10 @@ static int sriov_init(struct pci_dev *dev, int pos)
> if (!iov)
> return -ENOMEM;
>
> + /* Sizing SR-IOV BARs with VF Enable cleared - no decode */
> + __pci_size_stdbars(dev, PCI_SRIOV_NUM_BARS,
> + pos + PCI_SRIOV_BAR, sriovbars);
> +
> nres = 0;
> for (i = 0; i < PCI_SRIOV_NUM_BARS; i++) {
> res = &dev->resource[i + PCI_IOV_RESOURCES];
> @@ -796,7 +801,8 @@ static int sriov_init(struct pci_dev *dev, int pos)
> bar64 = (res->flags & IORESOURCE_MEM_64) ? 1 : 0;
> else
> bar64 = __pci_read_base(dev, pci_bar_unknown, res,
> - pos + PCI_SRIOV_BAR + i * 4);
> + pos + PCI_SRIOV_BAR + i * 4,
> + &sriovbars[i]);
> if (!res->flags)
> continue;
> if (resource_size(res) & (PAGE_SIZE - 1)) {
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 2e40fc63ba31..6f27651c2a70 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -315,8 +315,10 @@ bool pci_bus_generic_read_dev_vendor_id(struct pci_bus *bus, int devfn, u32 *pl,
> int pci_idt_bus_quirk(struct pci_bus *bus, int devfn, u32 *pl, int rrs_timeout);
>
> int pci_setup_device(struct pci_dev *dev);
> +void __pci_size_stdbars(struct pci_dev *dev, int count,
> + unsigned int pos, u32 *sizes);
> int __pci_read_base(struct pci_dev *dev, enum pci_bar_type type,
> - struct resource *res, unsigned int reg);
> + struct resource *res, unsigned int reg, u32 *sizes);
> void pci_configure_ari(struct pci_dev *dev);
> void __pci_bus_size_bridges(struct pci_bus *bus,
> struct list_head *realloc_head);
> 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
> @@ -164,41 +164,67 @@ 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 @@ int __pci_read_base(struct pci_dev *dev, enum pci_bar_type type,
>
> 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);
> }
> }
>
>