[Xen-devel] [PATCH 0/2] [RFC] xl: add cpuid config file option

[Andre Przywara <andre.przywara@xxxxxxx>]

Hi,

xl is currently ignoring the cpuid= variable in the config file. As I
don't like the current interface xm exposes (basically because it is
complicated, unintuitive and very error prone), I implemented a new
scheme for specifying CPUID flags policy, combining QEMU's and Xen's
approach:
cpuid = "<base>,<feature_name>=[01xks]*,...
The patch includes a (preliminary) list of feature names along with
their bit positions. The value for each feature bit copies the current
meaning is Xen:
0: clear, 1: set, x: don't care/use default, k: keep from host, s: use
host but preserve across migration
The value can also be a number (either in hex or decimal), so things
like "stepping=3" can be easily specified.
To show you the advantage, I quote the example config file:
> #cpuid=[ '1:ecx=xxxxxxxxxxx00xxxxxxxxxxxxxxxxxxx,
> #           eax=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx' ]
> # - Unset the SSE4 features (CPUID.1[ECX][20-19])
> # - Default behaviour for all other bits in ECX And EAX registers.
new version: cpuid = "host,sse4.1=0,sse4.2=0"
> #   Expose to the guest multi-core cpu instead of multiple processors
> # Example for intel, expose a 8-core processor :
> #cpuid=['1:edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx,
> #          ebx=xxxxxxxx00010000xxxxxxxxxxxxxxxx',
> #     '4,0:eax=001111xxxxxxxxxxxxxxxxxxxxxxxxxx']
> #  - CPUID.1[EDX][HT] : Enable HT
> #  - CPUID.1[EBX] : Number of vcpus * 2
> #  - CPUID.4,0[EAX] : Number of vcpus * 2 - 1
> #vcpus=8
new version: cpuid = "host,htt=1,proccount=16,maxcores=15"
> # Example for amd, expose a 5-core processor :
> # cpuid = ['1:ebx=xxxxxxxx00001010xxxxxxxxxxxxxxxx,
> #             edx=xxx1xxxxxxxxxxxxxxxxxxxxxxxxxxxx',
> # '0x80000001:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx1x',
> # '0x80000008:ecx=xxxxxxxxxxxxxxxxxxxxxxxxxx001001']
> #   - CPUID.1[EBX] : Threads per Core * Cores per Socket (2 * #vcpus)
> #   - CPUID.1[EDX][HT] : Enable HT
> #   - CPUID.0x80000001[CmpLegacy] : Use legacy method
> #   - CPUID.0x80000008[ECX] : #vcpus * 2 - 1
new version: cpuid="host,htt=1,cmplegacy=1,proccount=10,nc=9"

The parse_cpuid function in xl_cmdimpl.c parses the string and
translates it into the interface used by libxc.

If backward compatibility to the xm version is needed, I could also add
a quirk for the old interface. Since it uses a Python list, this can be
intercepted early in xl's parsing process and could use a compat code path.

This first version works for me, I'd like to hear your comments.

Regards,
Andre.

--
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12


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