Re: [Qemu-devel] [PATCH v8 1/4] firmware: introduce sysfs driver for QEMU's fw_cfg device

["Gabriel L. Somlo" <somlo@cmu.edu>]

On Tue, Feb 23, 2016 at 07:07:36AM +0200, Michael S. Tsirkin wrote:
> On Mon, Feb 22, 2016 at 03:26:23PM -0500, Gabriel L. Somlo wrote:
> > On Mon, Feb 22, 2016 at 10:14:50PM +0200, Michael S. Tsirkin wrote:
> > > On Sun, Feb 21, 2016 at 08:06:17AM -0500, Gabriel L. Somlo wrote:
> > > > > > +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 */

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

> > > > (I even had a prototype version doing what you suggested, but per the above
> > > > reference decided to drop it -- which IMHO is for the better, since otherwise
> > > > I'd have had to ifdef between ACPI and non-ACPI versions of the driver --
> > > > see https://lkml.org/lkml/2015/11/4/534 )
> > > 
> > > I'm not sure why you have these ifdefs - they are on the host, are they
> > > not?
> > 
> > Think of those as "pseudocode" ifdefs, they're there to distinguish
> > between AML that would be generated on MMIO vs. IOPORT systems
> > (specifically, arm vs. x86, respectively)
> > 
> > Some of the AML is the same, but obviously the _CRS, and
> > OperationRegion + Field are different, and I wanted to point that out
> > somehow :)
> > 
> > Cheers,
> > --Gabriel
> 
> You can do ifs as well.

Yeah, but the AML is generated from arch-specific locations in QEMU,
so we'd be doing MMIO-only from e.g. hw/arm/virt-acpi-build.c, and
IOPORT-only from hw/i386/acpi-build.c, etc. I wouldnt need to write a
generic AML blob with 'if' statements and insert it the same way on
all architectures, or would I ? Not sure what the best practice would
be for that :)

Speaking of AML, if we were to implement a "RDBL" (read-blob) method
for fw_cfg in AML, and call it from the guest-side kernel module,
we'll never be able to make it use DMA on ACPI systems. The way
fw_cfg_read_blob is written now, we could patch that in at some later
point. So that's an argument in favor of *at most* wrapping
acpi_os_acquire_mutex() around the current fw_cfg_read_blob, rather
than including an acpi-specific version implemented on top of an
AML call.

Thanks,
--Gabriel