Am 04.01.21 um 19:43 schrieb Alex Williamson:
On Mon, 4 Jan 2021 18:39:33 +0100
Christian König <christian.koenig@xxxxxxx> wrote:
Am 04.01.21 um 17:45 schrieb Alex Williamson:
On Mon, 4 Jan 2021 12:34:34 +0100
Christian König <christian.koenig@xxxxxxx> wrote:
[SNIP]
That's a rather bad idea. See our GPUs for example return way more than
they actually need.
E.g. a Polaris usually returns 4GiB even when only 2GiB are installed,
because 4GiB is just the maximum amount of RAM you can put together with
the ASIC on a board.
Would the driver fail or misbehave if the BAR is sized larger than the
amount of memory on the card or is memory size determined independently
of BAR size?
Uff, good question. I have no idea.
At least the Linux driver should behave well, but no idea about the
Windows driver stack.
Some devices even return a mask of all 1 even when they need only 2MiB,
resulting in nearly 1TiB of wasted address space with this approach.
Ugh. I'm afraid to ask why a device with a 2MiB BAR would implement a
REBAR capability, but I guess we really can't make any assumptions
about the breadth of SKUs that ASIC might support (or sanity of the
designers).
It's a standard feature for FPGAs these days since how much BAR you need
depends on what you load on it, and that in turn usually only happens
after the OS is already started and you fire up your development
environment.
We could probe to determine the maximum size the host can support and
potentially emulate the capability to remove sizes that we can't
allocate, but without any ability for the device to reject a size
advertised as supported via the capability protocol it makes me nervous
how we can guarantee the resources are available when the user
re-configures the device. That might mean we'd need to reserve the
resources, up to what the host can support, regardless of what the
device can actually use. I'm not sure how else to know how much to
reserve without device specific code in vfio-pci. Thanks,
Well in the FPGA case I outlined above you don't really know how much
BAR you need until the setup is completed.
E.g. you could need one BAR with just 2MiB and another with 128GB, or
two with 64GB or.... That's the reason why somebody came up with the
REBAR standard in the first place.
I think I can summarize that static resizing might work for some devices
like our GPUs, but it doesn't solve the problem in general.
Regards,
Christian.
Alex
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