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

Mitchell Augustin
2025-01-21T19:01:54Z

Thanks Alex,

I just verified that v2 of the patch causes no regressions and
performs just as well as v1 and my original patch on my hardware.

Reviewed-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
Tested-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>

On Mon, Jan 20, 2025 at 4:44 PM Bjorn Helgaas <helgaas@kernel.org> wrote:
>
> On Mon, Jan 20, 2025 at 11:21:59AM -0700, 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>
>
> Updated pci/enumeration with this v2, thanks, Alex!
>
> > ---
> >
> > 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.
> >
> > 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);
> > }
> > }
> >
> > --
> > 2.47.1
> >

--
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering