Re: [PATCH iwl-net v5] ixgbevf: fix link speed reporting for Hyper-V E610 VFs

[Przemek Kitszel <przemyslaw.kitszel@intel.com>]

On 8/26/26 16:39, Tomasz Lichwala wrote:
> When an E610 VF is running under Hyper-V, the VFLINKS register does not
> carry valid link speed. The existing code reads speed from VFLINKS, which
> does not reflect the actual negotiated speed. This results in ethtool
> reporting a stale or incorrect link speed.
> 
> The Hyper-V synthetic NIC exposes the actual link status through
> emulated PCI config space at offset 0x209 in VFLINKS register format.
> Read and decode link status from there when checking link on E610 VFs.
> 
> To avoid generating unnecessary VMBus transactions on every watchdog
> cycle, read the PCI config register on demand when ethtool or sysfs
> queries link speed, rather than polling it periodically. The cached
> value is used by the watchdog and link state notifications as before.
> 
> Guard the PCI config space read with IS_ENABLED(CONFIG_PCI_MMCONFIG),
> since accessing offsets above 256 requires MMCONFIG support.
> 
> Fixes: 4c44b450c69b ("ixgbevf: Add support for Intel(R) E610 device")
> Reviewed-by: Marcin Szycik <marcin.szycik@linux.intel.com>
> Signed-off-by: Tomasz Lichwala <tomasz.lichwala@linux.intel.com>

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>

> ---

next time please add something like "Notes for AI reviewer:" before such
notes :)

> Note on IS_ENABLED(CONFIG_PCI_MMCONFIG): this guard is kept intentionally
> for consistency with ixgbevf_hv_reset_hw_vf() which uses the same pattern
> to access emulated PCI config space at offset 0x201. Both functions read
> extended config space offsets (>256) that require MMCONFIG on x86. Hyper-V
> E610 VFs are currently only deployed on x86 where CONFIG_PCI_MMCONFIG is
> always enabled. If ARM64 Hyper-V support becomes relevant, both functions
> should be updated together.
> 
> Note on watchdog race: the mbx_lock serializes concurrent check_link calls
> so ethtool and watchdog cannot execute check_link simultaneously. The
> ethtool response reads adapter->link_speed immediately after check_link
> returns (same function, same thread) and always reports the fresh value.
> The watchdog's pre-existing pattern of caching adapter fields into locals
> before the lock applies to all HV VF types and is not introduced by this
> patch.
> 
> v5:
>   - Use subsystem device ID check instead of mac type to ensure the
>     on-demand ethtool read only triggers for Hyper-V VFs, not bare-metal