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Re: [Xen-devel] [PATCH v4] Hvmloader: Modify ACPI to only supply _EJ0 methods for PCIslots that support hotplug by runtime patching

On 05/12/2014 05:05 AM, Ian Campbell wrote:
On Fri, 2014-05-09 at 13:32 -0400, Ross Philipson wrote:
On 05/09/2014 12:34 PM, Paul Durrant wrote:
-----Original Message-----
From: Ian Campbell
Sent: 09 May 2014 17:12
To: Konrad Rzeszutek Wilk
Cc: Ross Philipson; kevin@xxxxxxxxxxxx; Huangweidong (C); Hanweidong
(Randy); mst@xxxxxxxxxx; qemu-devel@xxxxxxxxxx; xen-
devel@xxxxxxxxxxxxx; fabio.fantoni@xxxxxxx;
johannes.krampf@xxxxxxxxxxxxxx; Gonglei (Arei); Stefano Stabellini;
Gaowei (UVP); Jan Beulich; Anthony Perard; Paul Durrant
Subject: Re: [Xen-devel] [PATCH v4] Hvmloader: Modify ACPI to only supply
_EJ0 methods for PCIslots that support hotplug by runtime patching

On Fri, 2014-05-09 at 12:00 -0400, Konrad Rzeszutek Wilk wrote:

So we could just then gat the _EJ0 functionality based on values that
are present (or not) in the SSDT ?

AIUI the very presence of _EJ0 is what marks the device as being
ejectable (e.g. in the Windows device manager).

It would be possible to make _EJ0 conditionally turn itself into a NOP
without resorting to an SSDT, but I don't think that solves the issue
they are trying to solve, which is that the user can even try to eject
an non-hotplug device. (grep for UAR1 in our dsdt.asl and
acpi_info->com1_present in hvmloader/acpi/build.c for an example of this
sort of conditional thing)

Going back to the SSDT idea. A little poking around and what not and I
came up with something like this that I build into an SSDT:

DefinitionBlock ("SSDTX.aml", "SSDT", 2, "Xen", "HVM", 0)
      /* S00 device is defined in DSDT, this allows me to
       * refrence it in this SSDT
      External (\_SB.PCI0.S00, DeviceObj)


      /* Extend the functionality of S00 */
      Scope ( \_SB.PCI0.S00 ) {
          Method(_EJ0, 1, NotSerialized)
              /* Do stuffs here */

Thanks, this looks like the sort of thing I was naively imagining would
be possible.

So I did find some examples of this after all in my pile of ACPI
firmware snapshots from all our supported platforms.

Thanks (none of the machines I looked at had PCI hotplug apparently). I
was curious to know how Real Firmware Engineers(tm) dealt with this sort
of issue.

I was worried how real life OSPMs might interpret this method being in
an SSDT instead of the DSDT. In theory it shouldn't matter, and the fact
that real firmware does this seem to suggest that at least Windows
treats it that way (which is a relief).

I did actually find SSDTs that were specifically adding an _EJ0 to a device scope for a device defined externally. I attached an example from a Fujitsu system I have. The PRT1 device on SAT0 is external:

External (\_SB_.PCI0.SAT0.PRT1, DeviceObj)

And _EJ0 is added to the scope.

  I think this would
work allowing you to just add or not add _EJ0 methods to the PCI devices
you want by either using different SSDTs or doing something to generate
or munge the SSDT at runtime (which would be simpler than messing with
the DSDT I think.

Without filling out the body of _EJ0 (which I tried but failed to do)
your stub compiles to 60 bytes of AML, I suppose that even having filled
in _EJ0 in the result would be less than, say, 128 bytes.

Given that there are 32 PCI slots we would be talking about a total of
4k of space in hvmloader to provide a precompiled SSDT for each slot,
which can be inserted at runtime depending on each slots configuration.

I wouldn't be especially surprised if the code to generate a suitable
SSDT dynamically was a reasonable proportion of that size, so unless
there is the possibility of needing other variants it seems like just
generating each of them would be the say to go.

  I did not try it (actually I did but ran into other
problems on our platform :).



Ross Philipson

Attachment: SSDT2.dsl
Description: Text Data

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