PCI: epf-test: Improve handling of command and status registers

PCI: epf-test: Improve handling of command and status registers

The pci-epf-test driver uses the test register BAR memory directly to get
and execute a test registers set by the RC side and defined using a struct
pci_epf_test_reg. This direct use relies on using the register BAR address
as a pointer to a struct pci_epf_test_reg to execute the test case and to
send back the test result through the status field of struct
pci_epf_test_reg. In practice, the status field is always updated before an
interrupt is raised in pci_epf_test_raise_irq(), to ensure that the RC side
sees the updated status when receiving an interrupt.

However, such assignment direct access does not ensure that changes to the
status register make it to memory, and so visible to the host, before an
interrupt is raised, thus potentially resulting in the RC host not seeing
the correct status result for a test.

Avoid this potential problem by using READ_ONCE()/WRITE_ONCE() when
accessing the command and status fields of a pci_epf_test_reg structure.
This ensure that a test start (pci_epf_test_cmd_handler() function) and
completion (with the function pci_epf_test_raise_irq()) achieve a correct
synchronization with the MMIO register accesses on the RC host.

Link: https://lore.kernel.org/r/20230415023542.77601-10-dlemoal@kernel.org
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>