Re: [RFC PATCH v2 5/6] hw/arm/virt-acpi-build: Add PPTT table

["wangyanan (Y)" <wangyanan55@huawei.com>]

Hi Drew,

On 2021/5/19 16:27, Andrew Jones wrote:
> On Tue, May 18, 2021 at 09:05:39PM +0200, Andrew Jones wrote:
>> The problem is that -smp 4,maxcpus=8 doesn't error out today, even though
>> it doesn't do anything. OTOH, -smp 4,cores=2 doesn't error out either, but
>> we're proposing that it should. Maybe we can start erroring out when
>> cpus != maxcpus until hot plug is supported?
>>
> The more I think about this, the more I think we're in a bit of pickle and
> need Peter Maydell to chime in. While we may want to make our -smp command
> line option parsing more strict in order to bring some sanity to it, if
> we do, then we'll break existing command lines, which, while may be
> specifying useless inputs, have always gotten away with it. We probably
> can't just change that now without forcing the user to opt into it.
> Maybe we need to add another -smp parameter like 'strict' that has to
> be set to 'on' in order to get this new behavior.
>
> Peter, do you have some suggestions for this? A summary of the problem
> we'd like to solve is as follows:
>
>   We'd like to start describing CPU topology to guests when provided
>   topology information with the '-smp ...' command line option. Currently,
>   a user may provide nearly whatever it wants on that command line option
>   and not get an error, even though the guest will not get a topology
>   description. When building the topology its important to know what
>   the user actually wants, so we're proposing to require both sockets
>   and cores be given if one of them is given. Also, since we don't yet
>   support hot plug for AArch64, we're proposing to enforce cpus == maxcpus.
>
> Is it fine to make those changes to the parsing for 6.1 and later? (Note,
> mach-virt will override the default smp_parse with its own, so this is
> mach-virt specific.) Or, should we only do this if a new parameter is
> also given, e.g. 'strict'. Something like
>
>    -smp strict=on,cpus=4,sockets=2,cores=2
>
> would be needed by users who want to describe cpu topologies. Without
> a strict description, then they get what they get today for their
> DT/ACPI topology description, nothing.
 From my point of view, I like the idea of a new parameter like 
"strict=on/off".
I will explain the reason below but maybe I have missed something, so I also
hope for some suggestions from Peter. :)

1) We don't need to worry about breaking any existing -smp command lines
including the rare and strange ones any more, since we will only have more
strict requirement for the new provided cmdlines with "strict=on" and only
generate topology description to guest with these new cmdlines provided.

2) This will provide an option for users to decide whether to enable the 
feature
or not. Furthermore, this feature can also work on older machine types, 
if a user
want to make use of cpu topology exposure to guest on older machines and is
also sure it won't affect the application's behavior, then he can read 
the Doc and
properly provided a -smp cmdline with "strict=on" to boot a VM.

3) We don't need to bother guessing different formats of -smp command lines
in parsing. If the new parameter is not specified or "strict=off" is 
provided, we
totally follow the rules in smp_parse() and disable the topology 
exposure. And if
"strict=on" is provided, we enable the topology exposure and enforce 
completely
detailed configuration like "-smp strict=on,cpus=4,sockets=2,cores=2".

But maxcpus will be optional, it will default to cpus if not provided. 
We also ensure
it matches cpus if provided, given that cpu hotplug is not available yet.

Thanks,
Yanan
> Thanks,
> drew
>
> .