On Tue, Feb 23, 2016 at 04:14:46PM +0200, Michael S. Tsirkin wrote:

On Tue, Feb 23, 2016 at 08:47:00AM -0500, Gabriel L. Somlo wrote:

quoted

On Tue, Feb 23, 2016 at 07:07:36AM +0200, Michael S. Tsirkin wrote:

quoted

On Mon, Feb 22, 2016 at 03:26:23PM -0500, Gabriel L. Somlo wrote:

quoted

On Mon, Feb 22, 2016 at 10:14:50PM +0200, Michael S. Tsirkin wrote:

quoted

On Sun, Feb 21, 2016 at 08:06:17AM -0500, Gabriel L. Somlo wrote:

quoted

quoted

quoted

+static void fw_cfg_io_cleanup(void)
+{
+	if (fw_cfg_is_mmio) {
+		iounmap(fw_cfg_dev_base);
+		release_mem_region(fw_cfg_p_base, fw_cfg_p_size);
+	} else {
+		ioport_unmap(fw_cfg_dev_base);
+		release_region(fw_cfg_p_base, fw_cfg_p_size);
+	}
+}
+
+/* arch-specific ctrl & data register offsets are not available in ACPI, DT */

So for all arches which support ACPI, I think this driver
should just rely on ACPI.

There was a discussion about that a few versions ago, and IIRC the
conclusion was not to expect the firmware to contend for fw_cfg access
after the guest kernel boots:

https://lkml.org/lkml/2015/10/5/283

So it looks like NVDIMM at least wants to pass label data to guest -
for which fw cfg might be a reasonable choice.

I suspect things changed - fw cfg used to be very slow but we now have
DMA interface which makes it useful for a range of applications.

Comment on this? I'm really worried we'll release linux
without a way to access fw cfg from aml.
How about taking acpi lock around all accesses?

You mean something like this (haven't tried compiling it yet, so it
might be a bit more complicated, but just for the purpose of this
conversation):

diff --git a/drivers/firmware/qemu_fw_cfg.c

b/drivers/firmware/qemu_fw_cfg.c
index fedbff5..3462a2c 100644

--- a/drivers/firmware/qemu_fw_cfg.c
+++ b/drivers/firmware/qemu_fw_cfg.c

@@ -77,12 +77,18 @@ static inline u16 fw_cfg_sel_endianness(u16 key)
 static inline void fw_cfg_read_blob(u16 key,
                                    void *buf, loff_t pos, size_t

count)
 {
+#ifdef CONFIG_ACPI
+       acpi_os_acquire_mutex(acpi_gbl_osi_mutex, ACPI_WAIT_FOREVER);
+#endif
        mutex_lock(&fw_cfg_dev_lock);
        iowrite16(fw_cfg_sel_endianness(key), fw_cfg_reg_ctrl);
        while (pos-- > 0)
                ioread8(fw_cfg_reg_data);
        ioread8_rep(fw_cfg_reg_data, buf, count);
        mutex_unlock(&fw_cfg_dev_lock);
+#ifdef CONFIG_ACPI
+       acpi_os_release_mutex(acpi_gbl_osi_mutex);
+#endif
 }
 
 /* clean up fw_cfg device i/o */

Fundamentally yes.

quoted

I wouldn't particularly *mind* doing that, but I'd still like to hear
from other QEMU devs on whether it's really necessary.

It seems like a prudent thing to do IMHO, before this
goes out to users.

[...]

On balance, I think locking ACPI solves most problems so
if we just do that, I think what you did here is fine.

Only trouble is, acpi_gbl_osi_mutex seems to be "private" to the acpi
subsystem, and I'm not sure how well a patch to allow some random
module to lock/unlock ACPI at its convenience would be received...

So unless I'm missing something obvious (wouldn't be the first time),
I think we're back to where *if* we *have* to do this [*], providing an
AML blob-reader method in ACPI and punting to it from the guest-side
kernel module (via #ifdef CONFIG_ACPI) would be the less painful
alternative.

[*] that is, mutual exclusion between kernel and firmware regarding
    fw_cfg is (back) on the table, for real this time...

It would be good to know that it's the new consensus among QEMU
folks, since I have a strong feeling I'd no longer be "Keeping It Simple"
by moving in this direction.

Thanks,
--Gabriel