Re: [kvm-unit-tests PATCH v2] x86: pci: Support unaligned register access in PCI config read/write

[David Matlack <dmatlack@google.com>]

On 2026-08-04 11:23 PM, Irene Wang wrote:
> Per the PCI Local Bus Specification (Section 3.2.2.3.2, "Configuration
> Mechanism #1"), port 0xCF8 (CONFIG_ADDRESS) requires a DWORD-aligned
> register offset (bits [1:0] = 00b), while byte and word offsets within
> the DWORD must be selected via data port 0xCFC + (reg & 3).
> 
> Previously, PCI_CONF1_ADDRESS did not clear bits [1:0], and 8-bit /
> 16-bit helpers always accessed base port 0xCFC directly. This worked
> in QEMU because its PCI host bridge emulation preserves unaligned bits
> in CONFIG_ADDRESS and uses them during CONFIG_DATA accesses. However,
> in strictly spec-compliant VMMs (and potentially real hardware), bits
> [1:0] of 0xCF8 are ignored, causing non-aligned reads/writes at port
> 0xCFC to erroneously target byte 0 of the DWORD. In practice, this
> causes pci_find_dev() to read the vendor ID twice instead of reading
> the vendor ID and device ID.
> 
> Fix this by masking `reg` with `~3` in PCI_CONF1_ADDRESS and adding
> the `(reg & 3)` offset to the CONFIG_DATA port for 8-bit and 16-bit
> accessors, ensuring compatibility across QEMU and other VMMs. Additionally,
> assert that 16-bit accesses do not use an offset of 3, as reading/writing a
> word across DWORD boundaries (port 0xCFC + 3) is invalid per spec.
> 
> Assisted-by: Gemini:gemini-3.6-flash
> Signed-off-by: Irene Wang <yiranirenewang@gmail.com>
> ---
> v1 -> v2:
>  - Enclose macro parameter `dev` in parentheses in PCI_CONF1_ADDRESS.
>  - Add assert((reg & 3) != 3) in pci_config_readw() and pci_config_writew()
>    to guard against illegal 16-bit accesses crossing DWORD boundaries.
> 
> Link to v1: https://lore.kernel.org/kvm/20260730182053.905335-1-yiranirenewang@gmail.com/
> 
>  lib/x86/asm/pci.h | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/x86/asm/pci.h b/lib/x86/asm/pci.h
> index 03e55c27..da0b59a3 100644
> --- a/lib/x86/asm/pci.h
> +++ b/lib/x86/asm/pci.h
> @@ -9,18 +9,19 @@
>  #include "pci.h"
>  #include "x86/asm/io.h"
>  
> -#define PCI_CONF1_ADDRESS(dev, reg)	((0x1 << 31) | (dev << 8) | reg)
> +#define PCI_CONF1_ADDRESS(dev, reg)	((0x1 << 31) | ((dev) << 8) | ((reg) & ~3))

A comment here would be useful. Spec-compliant host bridges (including
emulations) should ignore the lower 2 bits so clearing it should
technically be unnecessary. The reason we have to clear it is because
QEMU is not spec-compliant and will consume those 2 bits.

>  
>  static inline uint8_t pci_config_readb(pcidevaddr_t dev, uint8_t reg)
>  {
>      outl(PCI_CONF1_ADDRESS(dev, reg), 0xCF8);
> -    return inb(0xCFC);
> +    return inb(0xCFC + (reg & 3));
>  }
>  
>  static inline uint16_t pci_config_readw(pcidevaddr_t dev, uint8_t reg)
>  {
> +    assert((reg & 3) != 3);
>      outl(PCI_CONF1_ADDRESS(dev, reg), 0xCF8);
> -    return inw(0xCFC);
> +    return inw(0xCFC + (reg & 3));
>  }
>  
>  static inline uint32_t pci_config_readl(pcidevaddr_t dev, uint8_t reg)
> @@ -33,14 +34,15 @@ static inline void pci_config_writeb(pcidevaddr_t dev, uint8_t reg,
>                                       uint8_t val)
>  {
>      outl(PCI_CONF1_ADDRESS(dev, reg), 0xCF8);
> -    outb(val, 0xCFC);
> +    outb(val, 0xCFC + (reg & 3));
>  }
>  
>  static inline void pci_config_writew(pcidevaddr_t dev, uint8_t reg,
>                                       uint16_t val)
>  {
> +    assert((reg & 3) != 3);

Should assert((reg & 3) == 0) be added to the readl/writel routines as
well?

>      outl(PCI_CONF1_ADDRESS(dev, reg), 0xCF8);
> -    outw(val, 0xCFC);
> +    outw(val, 0xCFC + (reg & 3));
>  }
>  
>  static inline void pci_config_writel(pcidevaddr_t dev, uint8_t reg,
> -- 
> 2.55.0.571.g244d577d93-goog
>