Re: APIC_ID in apic_reg_write()

[Jan Kiszka <jan.kiszka@siemens.com>]

Am 2015-04-29 um 20:54 schrieb Bandan Das:
> Jan Kiszka <jan.kiszka@siemens.com> writes:
> 
>> Am 2015-04-29 um 18:47 schrieb Bandan Das:
>>>
>>> Why do we allow writes to APIC_ID ? On all _newer_ processors, it's
>>> read only. The spec doesn't explicitly mention it though, or atleast
>>> I couldn't find it. Does userspace have a reason to modify it ?
>>
>> The APIC ID is read-only for x2APIC. It remains R/W for xAPIC.
> 
> Are you sure ? In 10.4 of the SDM, there is Note that says
> "In processors based on Intel microarchitecture code name Nehalem the Local APIC ID
> Register is no longer Read/Write; it is Read Only."

Indeed - confusing, specifically as it is placed there without a direct
link to the corresponding row in the table. And it doesn't say "since"
or so.

> 
> And I can verify on a SandyBridge and Haswell system that it's RO there too.

So the APIC just ignores the writes, it doesn't through #GP at least.

> 
> In fact, that was one of the reasons I had submitted a patch to remove
> verify_local_APIC() from x86/kernel/apic.c (4399c03c678) If I am wrong we need to
> revert atleast the associated commit message :)

Well, we can't remove APIC ID modification support from KVM, though,
because older CPU types we may want to emulate correctly had that
feature. But we may have to make it configurable to ensure accurate
behaviour.

Jan

-- 
Siemens AG, Corporate Technology, CT RTC ITP SES-DE
Corporate Competence Center Embedded Linux