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Re: [Xen-devel] [RFC PATCH v2 03/26] ARM: GICv3 ITS: allocate device and collection table
Hi Andre,
On 20/01/2017 17:05, Andre Przywara wrote:
On 20/01/17 12:18, Julien Grall wrote:
Hi Andre,
On 20/01/2017 12:27, Andre Przywara wrote:
On 20/01/17 11:12, Julien Grall wrote:
Hello,
On 04/01/2017 22:47, Stefano Stabellini wrote:
On Thu, 22 Dec 2016, Andre Przywara wrote:
Each ITS maps a pair of a DeviceID (usually the PCI b/d/f triplet) and
an EventID (the MSI payload or interrupt ID) to a pair of LPI number
and collection ID, which points to the target CPU.
This mapping is stored in the device and collection tables, which
software
has to provide for the ITS to use.
Allocate the required memory and hand it the ITS.
The maximum number of devices is limited to a compile-time constant
exposed in Kconfig.
Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
xen/arch/arm/Kconfig | 6 +++
xen/arch/arm/gic-its.c | 114
++++++++++++++++++++++++++++++++++++++++++
xen/arch/arm/gic-v3.c | 5 ++
xen/include/asm-arm/bitops.h | 1 +
xen/include/asm-arm/gic-its.h | 51 ++++++++++++++++++-
5 files changed, 176 insertions(+), 1 deletion(-)
diff --git a/xen/arch/arm/Kconfig b/xen/arch/arm/Kconfig
index a7d941c..a369305 100644
--- a/xen/arch/arm/Kconfig
+++ b/xen/arch/arm/Kconfig
@@ -59,6 +59,12 @@ config MAX_HOST_LPI_BITS
This can be overriden on the command line with the
max_lpi_bits
parameter.
+config MAX_ITS_PCI_BUSSES
+ depends on HAS_ITS
+ int "Number of PCI busses the ITS supports (4)"
+ range 1 1024
+ default "4"
Given that any kind of devices can be behind an ITS, including non-PCI
devices, I think it is best to call this MAX_PHYS_ITS_DEVICES.
I don't think we should limit the number of devices supported at
compilation time. We aim to have a single Xen binary running on any
board. The best would be to guess the number at runtime, but I would be
fine with a command line options.
Yes, so I changed this already to specify the number of devices and will
now also add a command line option.
So do you want to get rid of the Kconfig entirely to just go with
#define-ing the default value in the code?
Or do we still want to have this easily accessible way of changing the
default, at least for the first time to simplify experimentation?
Before answering to your questions, I'd like to understand why you need
to have this value hardcoded or coming from the command line? Can't this
be figured out at runtime?
Also what is this issue to use the number of ID bits presented in TYPER?
Is it because your series is only handling flat table, so a large amount
of memory would be used?
My understanding is that is the TYPER value just specifies the number of
device ID bits the ITS supports, without necessarily requiring software
to allocate tables to cover all theoretically possible devices. Instead
the expectation is that an ITS driver has some clue about how many
devices the system has or will need - which is not true in the (current)
Xen case, I am afraid.
Looking at Linux code (see its_parse_baser_device), the number of bits
in TYPER is used to computer the number of device supported. The number
of device supported will be clamped if it is too big.
I think it is a quite sensible solution as a user may not know the last
DeviceID.
A good ITS implementation might just implement all architected 32 bits,
which would require 32GB of a flat device table :-(
Also the indirect table feature is optional - and it happens to be that
it is _not_ available on the Cavium ThunderX, IIRC, despite it having
sparse device IDs.
And Linux limits all BASER tables to be at most 16MB.
Do you know why the 16MB?
So I think having some kind of upper limit would be good to have in any
case, whether this has to be a compile time option or not is another
question.
Could not we have a size handling all the tables rather than being
specific to the device table?
Cheers,
--
Julien Grall
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